Multi-mass mems motion sensor

ABSTRACT

A micro-electro-mechanical system (MEMS) motion sensor is provided that includes a MEMS wafer having a frame structure, a plurality of proof masses suspended to the frame structure, movable in three dimensions, and enclosed in one or more cavities. The MEMS sensor includes top and bottom cap wafers bonded to the MEMS wafer and top and bottom electrodes provided in the top and bottom cap wafers, forming capacitors with the plurality of proof masses, and being together configured to detect motions of the plurality of proof masses. The MEMS sensor further includes first electrical contacts provided on the top cap wafer and electrically connected to the top electrodes, and a second electrical contacts provided on the top cap wafer and electrically connected to the bottom electrodes by way of vertically extending insulated conducting pathways. A method for measuring acceleration and angular rate along three mutually orthogonal axes is also provided.

RELATED PATENT APPLICATION

This patent application incorporates by reference, in its entirety, andclaims priority to U.S. Provisional Patent Application No. 62/006,469,filed Jun. 2, 2014.

TECHNICAL FIELD

The general technical field relates to micro-electro-mechanical systems(MEMS), and more particularly, to a multi-mass MEMS motion sensor, to amethod of measuring linear acceleration and angular rate of rotationusing such a MEMS motion sensor, and to a method of manufacturing such aMEMS motion sensor.

BACKGROUND

Micro-electro-mechanical systems (MEMS) are an increasingly importantenabling technology. MEMS inertial sensors are used to sense changes inthe state of motion of an object, including changes in position,velocity, acceleration or orientation, and encompass devices such asaccelerometers, gyroscopes, vibrometers and inclinometers. Broadlydescribed, MEMS devices are integrated circuits (ICs) containing tinymechanical, optical, magnetic, electrical, chemical, biological, orother, transducers or actuators. MEMS devices can be manufactured usinghigh-volume silicon wafer fabrication techniques developed over the pastfifty years for the microelectronics industry. Their resulting smallsize and low cost make them attractive for use in an increasing numberof applications in a broad variety of industries including consumer,automotive, medical, aerospace, defense, green energy, industrial, andother markets.

MEMS devices, in particular inertial sensors such as accelerometers andangular rate sensors or gyroscopes, are being used in a steadily growingnumber of applications. As the number of these applications grow, thegreater the demand to add additional functionality and more types ofMEMS into a system chip architecture. Due to the significant increase inconsumer electronics applications for MEMS sensors such as optical imagestabilization (OIS) for cameras embedded in smart phones and tablet PCs,virtual reality systems and wearable electronics, there has been agrowing interest in utilizing such technology for more advancedapplications which have been traditionally catered to by much larger,more expensive and higher grade non-MEMS sensors. Such applicationsinclude single- and multiple-axis devices for industrial applications,inertial measurement units (IMUs) for navigation systems and attitudeheading reference systems (AHRS), control systems for unmanned air,ground and sea vehicles and for personal indoor GPS-denied navigation.These applications also may include healthcare/medical and sportsperformance monitoring and advanced motion capture systems for nextgeneration virtual reality. These advanced applications often requirelower bias drift and higher sensitivity specifications well beyond thecapability of existing consumer-grade MEMS inertial sensors on themarket. In order to expand these markets and to create new ones, it isdesirable and necessary that higher performance specifications bedeveloped. It is also necessary to produce a low cost and small sizesensor and/or MEMS inertial sensor-enabled system(s).

Advanced applications generally require lower bias drift and highersensitivity specifications beyond existing consumer-grade MEMS inertialsensors. However, given that MEMS motion sensors such as accelerometersand gyroscopes are typically much smaller than traditional mechanicalgyroscopes, they tend to be subject to higher mechanical noise anddrift. Also, since position and attitude are calculated by integratingthe acceleration and angular rate data, respectively, noise and driftlead to growing errors in position. The mechanical noise spectrumF_(noise) of a resonator can be expressed asF_(noise)=(4k_(B)Tmω₀/Q)^(1/2), where k_(B)T is the thermal noise, m isthe inertial mass, ω₀ is the resonant frequency, and Q is the resonancequality factor. The noise equivalent acceleration a_(noise) for anaccelerometer (F=ma) can therefore be expressed asa_(noise)=(4k_(B)Tω₀/mQ)^(1/2), while the noise equivalent angular ratefor a gyroscope (F=2mvΩ) is Ω_(noise)=(k_(B)T/Qω₀A²)^(1/2), where A isthe gyroscope drive amplitude. It follows that increasing the mass m andquality factor Q can reduce mechanical noise. Because large Q valuestend to make the design of sensing electronics more difficult andexpensive, increasing the mass m is generally more desirable. Mostmodern commercial inertial sensors use thin film micromachiningtechniques to fabricate the MEMS, leading to thin masses with thicknessranging from about 1 to about 40 micrometers (μm). For these types ofMEMS sensors, increasing the mass typically involves increasing thesurface area of the mass perpendicular to its thickness. In turn, suchan increase in the surface area of the mass can lead to larger chipfootprints and higher cost. Accordingly, various challenges still existfor the design of low-noise MEMS motion sensors.

SUMMARY

In accordance with an aspect, there is provided amicro-electro-mechanical system (MEMS) motion sensor including:

-   -   a MEMS wafer having opposed top and bottom sides and including a        frame structure, a plurality of proof masses, and a plurality of        spring assemblies each suspending a corresponding one of the        proof masses from the frame structure and enabling the        corresponding one of the proof masses to move along mutually        orthogonal first, second and third axes;    -   top and bottom cap wafers respectively bonded to the top and        bottom sides of the MEMS wafer, the top cap, bottom cap and MEMS        wafers being electrically conductive, the top cap wafer, bottom        cap wafer and frame structure together defining one or more        cavities each enclosing one or more of the plurality of proof        masses, each proof mass being enclosed in one of the one or more        cavities;    -   top and bottom electrodes respectively provided in the top and        bottom cap wafers and forming capacitors with the plurality of        proof masses, the top and bottom electrodes being together        configured to detect motions of the plurality of proof masses;    -   first and second sets of electrical contacts provided on the top        cap wafer, the first set of electrical contacts being        electrically connected to the top electrodes, and the second set        of electrical contacts being electrically connected to the        bottom electrodes by way of insulated conducting pathways        extending successively through the bottom cap wafer, the frame        structure of the MEMS wafer and the top cap wafer.

In some embodiments, the MEMS motion sensor is configured as asix-degree-of-freedom (6-DOF) motion sensor enabling three-axis linearacceleration and angular rate measurements.

In some embodiments, top and bottom electrodes form a plurality ofelectrode assemblies each electrode assembly including at least one pairof the top and/or bottom electrodes. The plurality of electrodeassemblies includes:

-   -   a first sensing electrode assembly associated with one or more        of the plurality of proof masses and configured to detect a        translational motion of the one or more proof masses associated        with the first sensing electrode assembly along the first axis,        the translational motion being indicative of a linear        acceleration along the first axis;    -   a second sensing electrode assembly associated with one or more        of the plurality of proof masses and configured to detect a        rotation of the one or more proof masses associated with the        second sensing electrode assembly about the third axis, the        rotation about the third axis being indicative of a linear        acceleration along the second axis;    -   a third sensing electrode assembly associated with one or more        of the plurality of proof masses and configured to detect a        rotation of the one or more proof masses associated with the        third sensing electrode assembly about the second axis, the        rotation about the second axis being indicative of a linear        acceleration along the third axis;    -   a first driving electrode assembly associated with and        configured to drive a motion of one or more associated ones of        the plurality of proof masses along the first axis at an        out-of-plane drive frequency;    -   a fourth sensing electrode assembly associated with the one or        more proof masses associated with the first driving electrode        assembly and configured to sense a Coriolis-induced, rocking        motion of the one or more proof masses associated with the first        driving electrode assembly along the third axis, the        Coriolis-induced, rocking motion along the third axis being        indicative of an angular rate about the second axis;    -   a fifth sensing electrode assembly associated with the one or        more proof masses associated with the first driving electrode        assembly and configured to sense a Coriolis-induced, rocking        motion of the one or more proof masses associated with the first        driving electrode assembly along the second axis, the        Coriolis-induced, rocking motion along the second axis being        indicative of an angular rate about the third axis;    -   a second driving electrode assembly associated with and        configured to drive a rocking motion of one or more associated        ones of the proof masses along one of the second and third axes        at an in-plane drive frequency; and    -   a sixth sensing electrode assembly associated with the one or        more proof masses associated with the second driving electrode        assembly and configured to sense a Coriolis-induced, rocking        motion of the one or more proof masses associated with the        second driving electrode assembly along the other one of the        second and third axes, the Coriolis-induced, rocking motion        along the other one of the second and third axes being        indicative of an angular rate about the first axis.

In some embodiments, the out-of-plane and in-plane drive frequencieseach range from 1 to 100 kilohertz, and each of the first, second andthird sensing electrode assemblies are configured to sense the motion ofthe one or more proof masses associated therewith at an accelerationsensing frequency that is less than between about 30 percent and 50percent of both the out-of-plane and in-plane drive frequencies.

In some embodiments, the cavity of each mass associated with at leastone of the first and second driving electrode assemblies is ahermetically sealed vacuum cavity.

In some embodiments, the plurality of proof masses consists of two proofmasses configured as follows:

-   -   a first proof mass associated with the first driving electrode        assembly and the first, second, third, fourth and fifth sensing        electrode assemblies; and    -   a second proof mass associated with the second driving electrode        assembly and the sixth sensing electrode assembly.

In some embodiments, the plurality of proof masses consists of threeproof masses configured as follows:

-   -   a first proof mass associated with the first driving electrode        assembly and the fourth and fifth sensing electrode assemblies;    -   a second proof mass associated with the second driving electrode        assembly and the sixth sensing electrode assembly; and    -   a third proof mass associated with the first, second and third        sensing electrode assemblies.

In some embodiments, the plurality of proof masses consists of fourproof masses configured as follows:

-   -   first and second proof masses both associated with the first        driving electrode assembly and the fourth and fifth sensing        electrode assemblies; and    -   third and fourth proof masses both associated with the second        driving electrode assembly and the sixth sensing electrode        assembly, at least one of the four proof masses being further        associated with the first, second and third sensing electrode        assemblies.

In some embodiments, the plurality of proof masses consists of fiveproof masses configured as follows:

-   -   first and second proof masses both associated with the first        driving electrode assembly and the fourth and fifth sensing        electrode assemblies; and    -   third and fourth proof masses both associated with the second        driving electrode assembly and the sixth sensing electrode        assembly; and    -   a fifth proof mass associated with the first, second and third        sensing electrode assemblies.

In some embodiments, the five proof masses are arranged in a commonplane encompassing the second and third axes, the fifth proof mass beinglocated centrally and surrounded by the first, second, third and fourthproof masses.

In some embodiments, the first driving electrode assembly is configuredto drive the first proof mass 180 degrees out-of-phase relative to thesecond proof mass, and the second driving electrode assembly isconfigured to drive the third proof mass 180 degrees out-of-phaserelative to the fourth proof mass.

In some embodiments of the MEMS motion sensor:

-   -   the fourth sensing electrode assembly is configured to form        first and second capacitors with the first and second proof        masses, respectively, and to measure a difference between a        capacitance of the first capacitor and a capacitance of the        second capacitor, the difference being indicative of an angular        rate of the first and second proof masses about the second axis;    -   the fifth sensing electrode assembly is configured to form third        and fourth capacitors with the first and second proof masses,        respectively, and to measure a difference between a capacitance        of the third capacitor and a capacitance of the fourth        capacitor, the difference being indicative of an angular rate of        the first and second proof masses about the third axis; and    -   the sixth sensing electrode assembly is configured to form fifth        and sixth capacitors with the third and fourth proof masses,        respectively, and to measure a difference between a capacitance        of the fifth capacitor and a capacitance of the sixth capacitor,        the difference being indicative of an angular rate of the third        and fourth proof masses about the first axis.

In some embodiments of the MEMS motion sensor:

-   -   the first sensing electrode assembly comprises top and bottom        sensing electrodes respectively located above and below a        central region of the fifth proof mass;    -   the second sensing electrode assembly comprises a pair of top        sensing electrodes disposed along a line parallel to the second        axis, on opposite sides of the top sensing electrode of the        first sensing electrode assembly, and a pair of bottom sensing        electrodes disposed along a line parallel to the second axis, on        opposite sides of the bottom sensing electrode of the first        sensing electrode assembly; and    -   the third sensing electrode assembly comprises a pair of top        sensing electrodes disposed along a line parallel to the third        axis, on opposite sides of the top sensing electrode of the        first sensing electrode assembly, and a pair of bottom sensing        electrodes disposed along a line parallel to the third axis, on        opposite sides of the bottom sensing electrode of the first        sensing electrode assembly.

In some embodiments of the MEMS motion sensor:

-   -   the first driving electrode assembly includes:        -   a pair of top driving electrodes, one of which being located            above a central region of the first proof mass and the other            being located above a central region of the second proof            mass; and        -   a pair of bottom driving electrodes, one of which being            located below the central region of the first proof mass and            the other being located below the central region of the            second proof mass;    -   the fourth sensing electrode assembly includes:        -   a first pair of top sensing electrodes disposed along a line            parallel to the third axis, on opposite sides of the top            driving electrode located above the central region of the            first proof mass;        -   a second pair of top sensing electrodes disposed along a            line parallel to the third axis, on opposite sides of the            top driving electrode located above the central region of            the second proof mass;        -   a first pair of bottom sensing electrodes disposed along a            line parallel to the third axis, on opposite sides of the            bottom driving electrode located below the central region of            the first proof mass; and        -   a second pair of bottom sensing electrodes disposed along a            line parallel to the third axis, on opposite sides of the            bottom driving electrode located below the central region of            the second proof mass; and    -   the fifth sensing electrode assembly includes:        -   a first pair of top sensing electrodes disposed along a line            parallel to the second axis, on opposite sides of the top            driving electrode located above the central region of the            first proof mass;        -   a second pair of top sensing electrodes disposed along a            line parallel to the second axis, on opposite sides of the            top driving electrode located above the central region of            the second proof mass;        -   a first pair of bottom sensing electrodes disposed along a            line parallel to the second axis, on opposite sides of the            bottom driving electrode located below the central region of            the first proof mass; and        -   a second pair of bottom sensing electrodes disposed along a            line parallel to the second axis, on opposite sides of the            bottom driving electrode located below the central region of            the second proof mass.

In some embodiments of the MEMS motion sensor:

-   -   the second driving electrode assembly includes:        -   a first pair of top driving electrodes disposed along a line            parallel to the one of the second and third axes, above and            laterally offset with respect to a central region of the            third proof mass;        -   a second pair of top driving electrodes disposed along a            line parallel to the one of the second and third axes, above            and laterally offset with respect to a central region of the            fourth proof mass;        -   a first pair of bottom driving electrodes disposed along a            line parallel to the one of the second and third axes, below            and laterally offset with respect to the central region of            the third proof mass; and        -   a second pair of bottom driving electrodes disposed along a            line parallel to the one of the second and third axes, below            and laterally offset with respect to the central region of            the fourth proof mass; and    -   the sixth sensing electrode assembly includes:        -   a first pair of top sensing electrodes disposed along a line            parallel to the other one of the second and third axes,            above and laterally offset with respect to a central region            of the third proof mass;        -   a second pair of top sensing electrodes disposed along a            line parallel to the other one of the second and third axes,            above and offset with respect to a central region of the            fourth proof mass;        -   a first pair of bottom sensing electrodes disposed along a            line parallel to the other one of the second and third axes,            below and laterally offset with respect to a central region            of the third proof mass; and        -   a second pair of bottom sensing electrodes disposed along a            line parallel to the other one of the second and third axes,            below and offset with respect to a central region of the            fourth proof mass.

In some embodiments, each proof mass and corresponding spring assemblyform a resonant structure configured to provide matched or near-matchedresonance conditions for angular rate measurements.

In some embodiments, the top cap wafer, bottom cap wafer and MEMS waferare each made of a silicon-based semiconductor.

In some embodiments, the MEMS wafer is a silicon-on-insulator (SOI)wafer including a device layer, a handle layer under and spaced from thedevice layer, and an insulating layer sandwiched between the device andhandle layers.

In some embodiments, the proof masses each have a thickness and apolygonal cross-section respectively along and perpendicular to thefirst axis, and the spring assemblies each include flexible springsmechanically connecting the corresponding proof mass to the framestructure, the flexible springs each having a thickness along the firstaxis that is significantly less than the thickness of the correspondingproof mass.

In some embodiments, the thickness of each of the plurality of proofmasses ranges from 10 to 1000 micrometers.

In accordance with another aspect, there is provided a MEMS motionsensor system architecture including:

-   -   a MEMS motion sensor as described herein; and    -   an integrated circuit (IC) wafer bonded to the top cap wafer of        the MEMS motion sensor, the IC wafer having circuitry        electrically connected to the MEMS motion sensor.

In some embodiments, the circuitry of the IC wafer is electricallyconnected to the first and second sets of electrical contacts of theMEMS motion sensor for routing signals to and from the top and bottomelectrodes.

In accordance with another aspect, there is provided a method ofmeasuring acceleration and angular rate along mutually orthogonal first,second and third axes.

The method includes:

-   -   (a) providing a MEMS motion sensor including a MEMS wafer having        opposed top and bottom sides and including a frame structure, a        plurality of proof masses, and a plurality of spring assemblies        each suspending a corresponding one of the proof masses from the        frame structure and enabling the corresponding one of the proof        masses to move along the first, second and third axes, and top        and bottom cap wafers respectively bonded to the top and bottom        sides of the MEMS wafer, the top cap, bottom cap and MEMS wafers        being electrically conductive, the top cap wafer, bottom cap        wafer and frame structure together defining one or more cavities        each enclosing one or more of the plurality of proof masses,        each proof mass being enclosed in one of the one or more        cavities;    -   (b) vibrating one or more of the proof masses along the first        axis at an out-of-plane drive frequency;    -   (c) sensing Coriolis-induced, rocking motions along the third        and second axes of the one or more proof masses driven along the        first axis, in response to an angular rate about the second and        third axes, respectively;    -   (d) vibrating one or more of the proof masses in a rocking        motion along one of the second and third axes at an in-plane        drive frequency;    -   (e) sensing a Coriolis-induced, rocking motion along the other        one of the second and third axes of the one or more proof masses        driven along the one of the second and third axes, in response        to an angular rate about the first axis; and    -   (f) sensing a translational motion along the first axis, a        rotation about the second axis, and a rotation about the third        axis of one of the proof masses, indicative of linear        accelerations along the first, third and second axes,        respectively.

In some embodiments, the plurality of proof masses consists of fiveproof masses, and:

-   -   step (b) includes vibrating first and second ones of the five        proof masses along the first axis at the out-of-plane drive        frequency;    -   step (c) includes sensing Coriolis-induced, rocking motions        along the third and second axes of the first and second proof        masses, in response to an angular rate of the first and second        proof masses about the second and third axes, respectively;    -   step (d) includes vibrating third and fourth ones of the five        proof masses in a rocking motion along the one of the second and        third axes at the in-plane drive frequency;    -   step (e) includes sensing a Coriolis-induced, rocking motion        along the other one of the second and third axes of the third        and fourth proof masses, in response to an angular rate of the        third and fourth proof masses about the first axis; and    -   step (f) includes sensing a translational motion along the first        axis, a rotation about the second axis, and a rotation about the        third axis of a fifth one of the five proof masses, indicative        of linear accelerations along the first, third and second axes        of the fifth proof mass, respectively.

In some embodiments, step (b) includes vibrating the first and secondproof masses 180 degrees out-of-phase with each other

In some embodiments, step (c) includes:

-   -   forming first and third capacitors with the first proof mass,        and second and fourth capacitors with the second proof mass;    -   measuring a difference between a capacitance of the first        capacitor and a capacitance of the second capacitor, the        difference being indicative of the angular rate of the first and        second proof masses about the second axis; and    -   measuring a difference between a capacitance of the third        capacitor and a capacitance of the fourth capacitor, the        difference being indicative of the angular rate of the first and        second proof masses about the third axis.

In some embodiments, step (d) includes vibrating the third and fourthproof masses 180 degrees out-of-phase with each other.

In some embodiments, step (e) includes:

-   -   forming fifth and sixth capacitors respectively with the third        and fourth proof masses;    -   measuring a difference between a capacitance of the fifth        capacitor and a capacitance of the sixth capacitor, the        difference being indicative of the angular rate of the third and        fourth proof masses about the first axis.

In some embodiments, step (f) includes sensing the translational motionalong the first axis, the rotation about the second axis, and therotation about the third axis of the fifth proof mass at respectiveacceleration sensing frequencies that are each less than between 30percent and 50 percent of both the out-of-plane and in-plane drivefrequencies.

In accordance with another aspect, there is provided a method formanufacturing a MEMS motion sensor. The method includes:

-   -   a) providing top and bottom cap wafers having respective inner        and outer sides; forming insulated conducting cap wafer channels        into the top and bottom cap wafers; patterning trenches and        filling the trenches to form respective top and bottom        electrodes on the inner sides of the top and bottom cap wafers,        at least a plurality of the insulated conducting cap wafer        channels being electrically connected to the top and bottom        electrodes;    -   b) providing a MEMS wafer having opposed top and bottoms sides,        and forming from one of the top and bottom sides, portions of a        plurality of proof masses, of flexible springs, and of a frame        structure with insulated conducting MEMS wafer channels;    -   c) bonding the one of the top and bottom sides of the MEMS wafer        to the inner side of the corresponding one of the top and bottom        cap wafers by aligning the insulated conducting cap wafer        channels with the corresponding portions of the insulated        conducting MEMS channels, and by aligning the electrodes with        the portions of the plurality of proof masses;    -   d) forming, from the other one of the top and bottom sides of        the MEMS wafer, remaining portions of the plurality of proof        masses, of the flexible springs, of the frame structure with the        insulated conducting MEMS wafer channels;    -   e) bonding the other one of the top and bottom sides of the MEMS        wafer to the inner side of the corresponding other one of the        top or bottom cap wafers by aligning the top electrodes with the        bottom electrodes and by aligning the insulated conducting cap        wafer channels of the corresponding other one of the top or        bottom cap wafers with the remaining portions of the insulated        conducting MEMS channels;    -   f) creating insulated conducting pathways, at least a plurality        of the insulated conducting pathways extending from the bottom        electrodes and successively through the frame structure of the        MEMS wafer and the top cap wafer;    -   g) enclosing the plurality of proof masses within corresponding        cavities defined by the top cap wafer, the bottom and bottom cap        wafers and by the outer frame of the MEMS wafer; and    -   h) removing top and bottom cap wafer material from the outer        sides of the top and bottom cap wafers, respectively to expose        and isolate the insulated conducting pathways and the top and        bottom electrodes.

Other features and advantages of the embodiments of the presentinvention will be better understood upon reading of preferredembodiments thereof with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, perspective view of a single-mass MEMS motionsensor.

FIG. 2 is a schematic, partially exploded perspective view of the MEMSmotion sensor of FIG. 1.

FIG. 3 is a schematic, cross-sectional view of the MEMS motion sensor ofFIG. 1, taken along section line III, depicting the position of theproof mass in the absence of acceleration and angular rate.

FIG. 4 is a schematic, cross-sectional view of the MEMS motion sensor ofFIG. 1, taken along section line III, depicting a rotation of the proofmass about the y axis in response to an acceleration along the x axis.

FIG. 5 is a schematic, cross-sectional view of the MEMS motion sensor ofFIG. 1, taken along section line III, depicting a translation of theproof mass about the z axis in response to an acceleration along the zaxis.

FIG. 6 is a schematic, cross-sectional view of the MEMS motion sensor ofFIG. 1, taken along section line III, depicting a Coriolis-induced,rocking motion of the proof mass along the x axis while being drivenalong the z axis, in response to the sensor being rotated around the yaxis.

FIG. 7 is a schematic, partial perspective view of the MEMS motionsensor of FIG. 1, depicting a Coriolis-induced, rocking motion of theproof mass along the y axis while being driven along the x axis, inresponse to the sensor being rotated around the z axis.

FIG. 8 is a schematic, perspective view of a multi-mass MEMS motionsensor including five proof masses, in accordance with an exemplaryembodiment.

FIG. 9 is a schematic, partially exploded perspective view of the MEMSmotion sensor of FIG. 8.

FIG. 10 is a schematic, cross-sectional view of the MEMS motion sensorof FIG. 8, taken along section line X.

FIG. 11 is a schematic, partial cross-sectional view of the MEMS motionsensor of FIG. 8, taken along section line XI, depicting the proof massused for linear acceleration measurements along the x, y and z axes.

FIG. 12 is a schematic, cross-sectional view of the MEMS motion sensorof FIG. 8, taken along section line X, depicting the pair of proofmasses and the sensing electrodes involved in-plane angular ratemeasurements.

FIG. 13 is a schematic, cross-sectional view of the MEMS motion sensorof FIG. 8, taken along section line XIII, depicting the drivingelectrodes driving the pair of proof masses 180 degrees out-of-phase forout-of-plane angular rate measurements.

FIGS. 14A and 14B are schematic, partial cross-sectional views of theMEMS motion sensor of FIG. 8, respectively taken along section linesXIVA and XIVB, depicting the pair of proof masses and the sensingelectrodes involved in out-of-plane angular rate measurements.

FIG. 15 is a schematic, cross-sectional view of the MEMS motion sensorof FIG. 8, taken along section line XV, depicting various insulatedconducting pathways extending downwardly from electrical contacts on thetop cap wafer.

FIGS. 16A to 16C are schematic, partially exploded perspective views ofthree other exemplary embodiments of a multi-mass MEMS motion sensorincluding two, three and four proof masses, respectively.

FIGS. 17A to 17P schematically illustrate steps of a method formanufacturing a multi-mass MEMS motion sensor, in accordance with anexemplary embodiment.

It should be noted that the appended drawings illustrate only exemplaryembodiments of the invention, and are therefore not to be construed aslimiting of its scope, for the invention may admit to other equallyeffective embodiments.

DETAILED DESCRIPTION

In the following description, similar features in the drawings have beengiven similar reference numerals, and, in order to preserve clarity inthe drawings, some reference numerals may be omitted when they werealready identified in a preceding figure. It should also be understoodthat the elements of the drawings are not necessarily depicted to scale,since emphasis is placed upon clearly illustrating the elements andstructures of the present embodiments.

General Overview—Multi-Mass MEMS Motion Sensor

The present description generally relates to a MEMS device implementedas a motion sensor having a plurality of pendulous proof massesdisplaceable along three mutually orthogonal axes and enablingmeasurements of linear acceleration along and angular rate of rotationabout the three mutually orthogonal axes. The present description alsogenerally relates to a method of measuring acceleration and angular rateusing such a MEMS motion sensor, as well as to a method of manufacturingsuch a MEMS motion sensor.

Throughout the present description, the term “motion sensor” refers to adevice or system capable of sensing at least linear acceleration andangular rate of rotation, but also possibly any of a number ofparameters indicative of a state of motion of an object, such asposition, velocity or orientation. In some implementations, the MEMSmotion sensor is configured as a six-degree-of-freedom (6-DOF)multi-mass motion sensor enabling three-axis linear acceleration andangular rate of rotation measurements.

Before describing exemplary embodiments of the multi-mass MEMS motionsensor, it is to be noted that 6-DOF capabilities can be achieved usinga single-mass MEMS motion sensor.

Referring to FIGS. 1 and 2, there are shown perspective and partiallyexploded perspective views illustrating different components of asingle-mass MEMS motion sensor 10′. The sensor 10′ includes a singleproof mass 17′ suspended to a frame structure 50′ by a spring assembly27′ that enables motion of the proof mass 17′ along mutually orthogonalfirst, second and third axes, designated as the z, x and y axes inFIG. 1. The proof mass 17′ and the spring assembly 27′ form a resonantstructure 54′ and are part of a central MEMS wafer 16′ sandwichedbetween top and bottom cap wafers 12′, 14′. The MEMS wafer 16′ can be asilicon-on-insulator (SOI) wafer including a device layer 20′, a handlelayer 22′, and an insulating layer 24′ (e.g., buried oxide) sandwichedbetween the device layer 20′ and the handle layer 22′. The motion sensor10 includes top and bottom electrodes 13′, 15′ respectively provided inthe top and bottom cap wafers 12′, 14′, and forming capacitors with theproof mass 17′. The top and bottom electrodes 13′, 15′ are configuredfor driving and sensing a motion of the proof mass 17. In FIGS. 1 and 2,the top and bottom electrodes 13′, 15′ each include five electrodes,namely a central electrode 13′a, 15 a′, two electrodes 13 b′, 13 c′, 15b′, 15 c′ disposed along the x axis on opposite sides of the centralelectrode, and two electrodes 13 d′, 13 e′, 15 d′, 15 e′ disposed alongthe y axis on opposite sides of the central electrode. An example ofsuch a single-mass MEMS motion sensor is provided in co-assignedinternational PCT patent application No PCT/CA2014/050730 filed Aug. 1,2014 and entit led “MEMS motion sensor and method of manufacturing”.

Referring to FIGS. 3 to 7, the 6-DOF capabilities of the single-massMEMS motion sensor 10′ of FIGS. 1 and 2 will be briefly described.

FIGS. 3 to 6 are cross-sectional views of the single-mass MEMS motionsensor 10′ of FIG. 1, taken along line 3. Broadly described, the motionof the singe-mass sensor 10′ can be sensed via differential capacitancemeasurements between the proof mass 17′ and one or more of the topelectrodes 13′ in the top cap wafer 12′ and/or one or more of the bottomelectrodes 15′ in the bottom cap wafer 14′. FIG. 3 illustrates that inabsence of acceleration and angular rate, the proof mass 17′ is ideallypositioned equidistant between the top and bottom electrodes 13′, 15′such that the differential capacitance is zero.

In FIG. 4, the single-mass sensor 10′ is subjected to acceleration a_(x)along the x axis, which causes the proof mass 17′ to rotate around thecenter of the resonant structure 54′ with an axis of rotation parallelto the y axis. This rotation about the y axis leads to a change indifferential capacitance proportional to the acceleration a_(x) alongthe x axis, for example by measuring the difference of the capacitancebetween the proof mass 17′ and the top electrode 13 b′ and thecapacitance between the proof mass 17′ and the top electrode 13 c′.Likewise, an acceleration a_(y) of the proof mass 17′ along the y axiscauses the proof mass 17′ to rotate around the x axis and can also besense as a variation in differential capacitance by two or more of theelectrodes 13 d′, 13 e′, 15 d′, 15 e′ disposed along the y axis (seeFIG. 2).

In FIG. 5, the single-mass MEMS motion sensor 10′ is accelerated alongthe z axis, which causes the proof mass 17′ to move vertically along thez axis. The acceleration a_(z) can be measured by monitoring thedifference of the capacitance between the proof mass 17′ and a topelectrode (e.g., 13 b′) and the capacitance between the proof mass 17′and the electrode diagonally opposite (e.g., 15 c′).

FIG. 6 illustrates the measurement of angular rate Ω_(y) about the yaxis using a measurement configuration that relies on the Corioliseffect. The Coriolis effect arises when a mass M moves at velocity{right arrow over (v)} in a reference frame rotating with angular rate{right arrow over (Ω)}. An observer sitting in the reference rotatingframe perceives the mass M to be deflected from its straight-linetrajectory by a fictitious force, the Coriolis force, given by {rightarrow over (F)}_(Coriolis)=2M{right arrow over (v)}×{right arrow over(Ω)}, where × denotes the vector cross-product. In FIG. 6, the proofmass 17′ is driven resonantly along the z axis. For example, the drivesignal can be a time-periodic signal given by z=z₀ sin(ωt), whichcorresponds to a velocity given by v_(z)=v_(z0) cos(ωt). If thesingle-mass sensor 10′ rotates around the y axis at an angular rate ofΩ_(y), the proof mass 17′ will oscillate, as a rocking motion along thex axis, in response to the Coriolis acceleration {right arrow over(a)}_(Coriolis)=2{right arrow over (v)}×{right arrow over(Ω)}=2v_(z0)Ω_(y) cos ωt{circumflex over (x)}=a_(Coriolis) cosωt{circumflex over (x)}. This motion can be measured as an oscillatingdifferential capacitance in a manner similar to the way the linearacceleration a_(x) is measured. The angular rate Ω_(x) about the x axiscan be sensed similarly by detecting a variation in differentialcapacitance in the signal measured by two or more of the electrodes 13d′, 13 e′, 15 d′, 15 e′ disposed along the y axis (see FIG. 2).

Finally, referring to FIG. 7, the angular rate Ω_(z) about the z axiscan be measured by driving the proof mass 17′ in a rocking motion alongone of the x and y axes, and by sensing a Coriolis-induced, rockingmotion of the proof mass 17′ along the other one of the x and y axes.

It is to be noted that although in some applications providing 6-DOFcapabilities using single-mass motion sensors may be possible ordesirable, compromises often have to be made in order to get anacceptable level of performance. For example, angular rate measurementstend to be more sensitive at a high resonant frequency (e.g., in therange from 10 to 20 kilohertz), less responsive to environmentalmechanical noise. In contrast, although linear accelerometers can beused to measure high frequencies (e.g., as vibrometers), generally onlylow frequencies from direct current (DC) to a few hundred hertz (e.g.,100 to 200 Hz bandwidth around 0 Hz) are useful for navigationapplications. Therefore, gyroscopes are typically operated in vacuumwith a high Q factor, while accelerometers are typical operated under anambient pressure to provide damping.

Another reason why multiple proof masses may, in some applications, bepreferred over single-mass 6-DOF MEMS motion sensors is the sensitivityof angular rate measurement to linear acceleration. Referring to FIGS. 4and 6, the measurement of the acceleration a_(x) along the x axis andthe measurement of the angular rate Ω_(y) about the y axis are generallymade using similar, if not identical, sensing electrodes. In most cases,the only characteristic that distinguishes the two measurements isangular rate is sensed at high (resonant) frequency, while linearacceleration is sensed at low frequency. Measuring the high-frequencyangular rate signal, a high-pass filter may be used to remove thelow-frequency acceleration signal. However, the low-frequency signal canbe several orders of magnitude larger than the angular rate signal sothat precision filters may be required to remove it.

A further reason why multiple proof masses may, in some applications, bepreferred over single-mass 6-DOF MEMS motion sensors is that when the xand y rocking frequencies are identical (e.g., for a symmetrical proofmass), it can be difficult to separate the signal of theCoriolis-induced, rocking motion along the y axis caused by a resonantdrive along the z axis and an angular rate about the x axis from thesignal of the Coriolis-induced, rocking motion along the y axis causedby a resonant drive along the x axis and an angular rate about the zaxis. Hence, for some applications, the sensor mechanics and electronicscould be made simpler if the out-of-plane (i.e., about the z axis) andin-plane (i.e., about the x and y axes) angular rate measurements arecarried out using different proof masses.

Therefore, there can be a number of mechanical and electronic reasonsand advantages to distribute the DOFs of motion and their associatedfunctions among multiple inertial proof masses, as will now bedescribed.

The MEMS motion sensor according to embodiments of the inventionincludes multiple proof masses integrated into a single device or chipusing a 3DTCV architecture that can enable wafer level integration ofMEMS and IC wafers. Additionally, the MEMS motion sensor may befabricated using a wafer-scale packaging scheme in which each of theproof masses is hermetically sealed in a cavity provided with a numberof electrodes above, below and/or around the proof mass to sense, and insome cases drive, its motion.

Broadly described, the MEMS motion sensor according to embodiments ofthe invention forms a multi-wafer stack that includes a MEMS waferhaving opposed top and bottom sides, and top and bottom cap wafersrespectively bonded to the top and bottom sides of the MEMS wafer. Thetop cap, bottom cap and MEMS wafer are electrically conductive. The MEMSmotion sensor also includes top and bottom electrodes respectivelyprovided in the top and bottom cap wafers, and first and second sets ofelectrical contacts provided on the top cap wafer. The MEMS waferincludes a frame structure, a plurality of proof masses, and a pluralityof spring assemblies each suspending a corresponding one of the proofmasses from the frame structure and enabling the corresponding one ofthe proof masses to move along mutually orthogonal first, second andthird axes. The top cap wafer, bottom cap wafer and frame structuretogether define one or more cavities. Each cavity encloses one or moreof the proof masses, and each proof mass is enclosed in one cavity. Thetop and bottom electrodes form capacitors with and are togetherconfigured to detect motions of the proof masses. The first set ofelectrical contacts are electrically connected to the top electrodes,while the second set of electrical contacts are electrically connectedto the bottom electrodes by way of electrically insulated conductingpathways extending successively through the bottom cap wafer, the framestructure of the MEMS wafer and the top cap wafer. The insulatedconducting pathways can also be referred to as “three-dimensionalthrough-chip vias” or “3DTCVs”.

The MEMS motion sensor according to embodiments of the invention can notonly enable encapsulation of the proof masses, but also make efficientuse of the protective top and bottom cap wafers by providing electrodesin the caps as well as insulated conducting pathways. These insulatedconducting pathways can allow signals to be routed from the bottom tothe top of the MEMS motion sensor where they can be accessed for signalprocessing, thus allowing for the electrical contacts to be providedonly on one side of the sensor. Of course, if needed or desired,electrical contacts may also be placed on the bottom side of the sensor.In some embodiments, the architecture of the MEMS motion sensor may alsoenable wire-bond-free electrical connection to an IC which can beflip-chip bonded to the top of the MEMS either at the chip or waferlevel, which can advantageously reduce complexity and cost of MEMS andIC integration and packaging.

In some embodiments, the MEMS motion sensor includes thick, bulk proofmasses to provide higher sensitivity, lower noise, and a larger area forcapacitive sensing than current two-dimensional MEMS gyroscopearchitecture fabricated with planar processes. Yet another advantage ofsome embodiments of the MEMS device resides in that the bulk proofmasses are each suspended from the frame structure by flexible springswhose thickness is significantly less than that of the correspondingproof mass, which enables separate adjustment of the proof mass andspring properties. In particular, in some embodiments, the proof masscan be rather thick, for example up to the thickness of a common siliconwafer (e.g., typically in the 400-700 μm range), while enabling thesprings to remain thin and flexible. In this particular example, thoseskilled in the art will recognize that a 400 μm thick spring could bequite stiff. Also, in such scenarios, the resonant frequencies can betuned by adjusting the width and thickness of the springs withoutmodifying the proof mass thickness.

First Exemplary Embodiment of a MEMS Motion Sensor: Five Proof Masses

Referring to FIGS. 8 to 10, there are depicted a perspective view, apartially exploded perspective view and a cross-sectional view of amulti-mass MEMS motion sensor 10, in accordance with an exemplaryembodiment. In the illustrated embodiment, the MEMS motion sensor 10 isimplemented as a 6-DOF motion sensor configured for three-axis linearacceleration and three-axis angular rate measurements. It is to be notedthat throughout the present description, and unless stated otherwise,the terms “MEMS device” and “MEMS motion sensor” are usedinterchangeably.

MEMS, Top Cap and Bottom Cap Wafers

In FIGS. 8 to 10, the MEMS motion sensor 10 first includes a MEMS wafer16 having opposed top and bottom sides 161, 162, and top and bottom capwafers 12, 14 respectively bonded to the top and bottom sides 161, 162of the MEMS wafer 16. The MEMS wafer 16 can be a SOI wafer including adevice layer 20, a handle layer 22 under and spaced from the devicelayer 20, and an insulating layer 24 (e.g., buried oxide) sandwichedbetween the device and handle layers 20, 22. In the illustratedembodiment, the top cap wafer 12 is bonded to and in electrical contactwith selected portions of the device layer 20, while the bottom capwafer 14 is bonded to and in electrical contact with selected portionsof the handle layer 22. The device layer 20 and the handle layer 22 ofthe MEMS wafer 16, as well as the top and bottom cap wafers 12, 14 aremade of electrically conductive material such as, for example, asilicon-based semiconductor material. The insulating layer 24 acts toinsulate the top half of the sensor 10 from the bottom half. In someimplementations, electrical shunts 30 can be formed through theinsulating layer 24 to allow electrical connections to be establishedbetween the device layer 20 and the handle layer 22 at desired places.

The MEMS wafer 16 includes a frame structure 50, a plurality of proofmasses 17 a to 17 e, and a plurality of spring assemblies 27 a to 27 e.Each spring assembly 27 a to 27 e suspends a corresponding one of theproof masses 17 a to 17 e from the frame structure 50 and enables thecorresponding one of the proof masses to move along mutually orthogonalfirst, second and third axes. As depicted in FIG. 8, the first, secondand third axes are designated for definiteness as the z, x and y axes,respectively, and will be so referred to herein. In particular, in theexemplary embodiment of FIGS. 8 to 10, the x and y axes may be referredto as “in-plane”, “lateral” or “horizontal” directions, while the z axismay be referred to as an “out-of-plane”, transverse” or “vertical”direction.

Also, throughout the present description, terms such as “top” and“bottom”, “above” and “below”, “over” and “under”, “upper” and “lower”,and other like terms indicating the position of one element with respectto another element are used herein for ease and clarity of description,as illustrated in the figures, and should not be considered limitative.It will be understood that such spatially relative terms are intended toencompass different orientations of the MEMS motion sensor in use oroperation, in addition to the orientation exemplified in the figures. Inparticular, the terms “top” and “bottom” are used to facilitate readingof the description, and those skilled in the art of MEMS will readilyrecognize that, when in use, MEMS devices can be placed in differentorientations such that, for example, the top and bottom cap wafers arepositioned upside down. It will further be understood that the term“over” and “under” in specifying the spatial relationship of one elementwith respect to another element denotes that the two elements are eitherin direct contact with or separated by one or more intervening elements.

Frame Structure, Proof Masses and Spring Assemblies

The term “frame structure” is intended to refer broadly to any structurethat holds and mechanically supports the proof masses such that theproof masses can move relative to the support assembly along the x, yand z axes. In FIGS. 8 to 10, the frame structure 50 is embodied by theportions of the MEMS wafer 16 spaced from and laterally enclosing theproof masses 17 a to 17 e. Also, the term “proof mass” is intended torefer broadly to any predetermined inertial mass used in a MEMS motionsensor whose motion can serve as a reference for a motion to be measured(e.g., linear acceleration, angular rate, and the like).

In the exemplary embodiment of FIGS. 8 to 10, the MEMS motion sensor 10is provided with five proof masses 17 a to 17 e, where the first andsecond proof masses 17 a, 17 b can be used for in-plane angular ratemeasurements around the x and y axes, the third and fourth proof masses17 c, 17 d can be used for out-of-plane angular rate measurements aroundthe z axis, and the fifth proof mass 17 e can be used for linearacceleration measurements along the x, y and z axes. In thisconfiguration, in-plane and out-of-plane angular rate measurements aremade using different pairs of proof masses. Also, by assigning the 3-DOFacceleration measurements to a separate proof mass, the sensitivity ofangular rate measurement to linear acceleration can be reduced.

In the illustrated embodiment, the five proof masses 17 a to 17 e arearranged in a common plane encompassing the x and y axes. In thisexemplary configuration, the fifth proof mass 17 e is located centrallyand surrounded by the other four proof masses 17 a to 17 d. Of course,various other symmetrical or non-symmetrical spatial arrangements of theproof masses can be used in other embodiments without departing from thescope of the present invention. Likewise, configurations with two,three, four or more than five proof masses could be employed in otherembodiments, as will discussed further below.

Each proof mass may be described in terms of a thickness and across-section respectively along and perpendicular to the z axis. Forexample, in the illustrated embodiment, each proof mass consists of anoctagonal central region provided with rectangular lobes extendingoutwardly from the central region along the x and y axes. Of course, inother embodiments, the proof masses may assume a variety of shapes,polygonal or not, and sizes which may but need not be the same for eachproof mass. For example, in some embodiments, the first and second proofmasses 17 a, 17 b used for x and y angular rate measurements may beidentical to each other by different from the other proof masses 17 c to17 e, and likewise for the third and fourth proof masses 17 c, 17 d usedfor z angular rate measurements.

As used herein, the term “spring assembly” is intended to refer to anysuitable flexible structure configured to suspend a proof mass in spacedrelationship from the frame structure and to enable motion of the proofmass relative to the frame structure along three mutually orthogonalaxes in response to a motion experienced by the MEMS motion sensor. InFIGS. 8 to 10, the spring assembly 27 a to 27 e of each proof mass 17 ato 17 e includes four flexible springs 270, each of which mechanicallyconnects and provides a restoring force between the frame structure 50and the corresponding proof mass 17 a to 17 e. Of course, the type,number, position and arrangement of the flexible springs can be variedin other embodiments. Likewise, in a given embodiment of the MEMS motionsensor, the flexible springs associated with different proof masses maybe different.

Each proof mass 17 a to 17 e and the corresponding spring assembly 27 ato 27 e may form a resonant structure 54 supporting a number ofoscillation modes, each characterized by one or more resonantfrequencies whose values can be set by adjusting the mechanical andgeometrical properties of the resonant structure (e.g., the width andthickness of the springs and the size and shape of the proof mass). Forexample, in the embodiment of FIGS. 8 to 10, the relative sizes of thelobes and central region of each proof mass can be varied in tandem withthe dimensions of the springs 270 to adjust the resonant frequencies ofthe resonant structure 54.

In FIGS. 8 to 10, the proof masses 17 a to 17 e are patterned in boththe device and handle layers 20, 22, while the spring assemblies 27 a to27 e are patterned only in the device layer 20. Accordingly, thethickness along the z axis of the flexible springs 270 of the springassemblies 27 a to 27 e can be made significantly less than thethickness of the corresponding proof masses 17 a to 17 e. For example,in some embodiments, the thickness of the proof masses 17 a to 17 e canbe greater than 10 μm up to as thick as the SOI MEMS wafer 16, whichtypically ranges from about 400 to 700 μm, and even up to 1000 μm,whereas the thickness of the flexible springs 270 can be of the order of1 to 10 μm. Therefore, in some embodiments, the ratio of the thicknessof one of the proof masses to the thickness of the correspondingflexible springs can be larger than 40. As mentioned above, providing aMEMS motion sensor in which the thickness of the proof massessignificantly exceeds that of the flexible springs can proveadvantageous. This is because the proof mass can consist of the entirethickness of the SOI MEMS wafer, while the springs can be kept thin and,thus, flexible enough to maintain the resonant frequencies at reasonablylow values (e.g., of the order of tens of kilohertz).

Cavities Enclosing the Proof Masses

Referring still to FIGS. 8 to 10, the top cap wafer 12, bottom cap wafer14 and frame structure 50 define, when bonded together, one or morecavities 31, or chambers, each of which encloses one or more of theproof masses 17 a to 17 e, such that each proof mass is enclosed withina cavity 31. In some embodiments, the pressure in each cavity 31 can beadjusted independently. In particular, in some embodiments, the proofmasses used for angular rate measurement are advantageously enclosed inrespective vacuum cavities. For example, in the illustrated embodiment,the cavities 31 enclosing the first, second, third and fourth proofmasses 17 a to 17 d, which are used for angular rate measurements, canbe hermetically sealed vacuum cavities, which may not be the case forthe fifth proof mass 17 e used for linear acceleration measurements.Also, although each proof mass is enclosed its own cavity in FIGS. 8 to10, in other embodiments, the number of cavities may be less than thenumber of proof masses. In particular, the number and arrangement of thecavities may be dictated by environmental (e.g., high or low pressure)and/or mechanical (e.g., support integrity) considerations.

Top and Bottom Electrodes

Referring still to FIGS. 8 to 10, the MEMS motion sensor 10 alsoincludes top and bottom electrodes 13, 15 respectively provided in thetop and bottom cap wafers 12, 14 and forming capacitors with theplurality of proof masses 17 a to 17 e. The top and bottom electrodes13, 15 are together configured to detect motions of the plurality ofproof masses, namely linear accelerations along and angular rate aboutthree mutually orthogonal axes. In some implementations, the top andbottom electrodes 13, 15 form eight electrode assemblies 181 to 188,where each of the electrode assemblies 181 to 188 includes at least onepair of electrodes. Each pair of electrodes is further composed of twoof the top electrodes 13 or two of the bottom electrodes 15 or one eachof the top and bottom electrodes 13, 15.

It will be understood that the subdivision of the top and bottomelectrodes 13, 15 into such electrode assemblies 181 to 188 is made froma functional or conceptual standpoint and that, in practice, a given“physical” top or bottom electrode 13, 15 may be part of more than oneelectrode assembly 181 to 188, and that the functions performed by twoor more electrode assemblies 181 to 188 may be performed by the same“physical” electrode 13, 15 without departing from the scope of thepresent invention.

Linear Acceleration Measurements

In order to provide three-axis linear acceleration sensing capabilities,the electrode assemblies 181 to 188 can include first, second and thirdsensing electrode assemblies 181 to 183, each associated with one ormore proof masses 17. In the embodiment of FIGS. 8 to 10, the first,second and third sensing electrode assemblies 181 to 183 are eachassociated with the fifth proof mass 17 e, which thus acts as a 3-DOFaccelerometer. In particular, the fifth proof mass 17 e is configured tomove vertically along the z axis in response to a z-directedacceleration and to rotate about the x and x axes in response to y- andx-directed accelerations, respectively. The first sensing electrodeassembly 181 is configured to detect a translational motion of the fifthproof mass 17 e along the z axis, the translational motion beingindicative of a linear acceleration along the z axis. The second sensingelectrode assembly 182 is configured to detect a rotation of the fifthproof mass 17 e about the y axis, the rotation being indicative of alinear acceleration along the x axis. The third sensing electrodeassembly 183 is configured to detect a rotation of the fifth proof mass17 e about the x axis, the rotation being indicative of a linearacceleration along the y axis.

Turning to FIGS. 9 and 11, in some embodiments, the first, second andthird sensing electrode assemblies 181 to 183 consist of four top andfour bottom sensing electrodes forming capacitors with the fifth proofmass 17 e and including:

-   -   a first pair of top sensing electrodes 13 a, 13 b disposed along        a line parallel to the x axis, above and laterally offset with        respect to a central region 171 of the fifth proof mass 17 e;    -   a first pair of top bottom sensing electrodes 15 a, 15 b        disposed along a line parallel to the x axis, below and        laterally offset with respect to a central region 171 of the        fifth proof mass 17 e;    -   a second pair of top sensing electrodes 13 c, 13 d disposed        along a line parallel to the y axis, above and laterally offset        with respect to a central region 171 of the fifth proof mass 17        e; and    -   a second pair of top bottom sensing electrodes 15 c, 15 d        disposed along a line parallel to the y axis, below and        laterally offset with respect to a central region 171 of the        fifth proof mass 17 e.

Of course, the number and arrangement of electrodes can vary dependingon the application in which the MEMS motion sensor is to be used.

For this electrode configuration, the acceleration a_(x), a_(y) anda_(z) can be determined using differential capacitance measurements. Forexample, by measuring the difference of the capacitance between thefifth proof mass 17 e and the electrode 13 a (or 15 a) and thecapacitance between the fifth proof mass 17 e and the electrode 13 b (or15 b), the displacement of the fifth proof mass 17 e along the z axis issubtracted out and a_(x) can be measured. The acceleration componenta_(y) can be obtained in a similar manner from the difference betweenthe capacitances measured by the electrode 13 c (or 15 c) and theelectrode 13 d (or 15 d). Furthermore, by taking the difference betweenthe capacitances measured by the electrode 13 a (or 13 b) and theelectrode 15 b (or 15 a), the displacement of the fifth proof mass 17 ealong the x axis is subtracted out and a_(z) can be measured.

Angular Rate Measurements

Referring back to FIGS. 8 to 10, in order to provide three-axis angularrate sensing capabilities, the electrode assemblies 181 to 188 caninclude first and second driving electrode assemblies 187, 188, andfourth, fifth and sixth sensing electrode assemblies 184 to 186, each ofthese driving and sensing electrode assemblies 183 to 188 beingassociated with one or more proof masses 17. In the embodiment of FIGS.8 to 10, the first driving and fourth and fifth sensing electrodeassemblies 187, 184, 185 are each associated with the second and thirdproof masses 17 a, 17 b, and are therefore used to measure in-planeangular rates about the x and y axes. Meanwhile, the second driving andsixth sensing electrode assemblies 188, 186 are each associated with thethird and fourth proof masses 17 c, 17 d, and are therefore used tomeasure out-of-plane angular rates about the z axis. In the illustratedembodiment, the electrodes associated with the first driving electrodeassembly 187 are located in corresponding above and below centralregions 171 of the first and second proof masses 17 a, 17 b, while theelectrodes associated with the second driving and fourth, fifth andsixth electrode assemblies 188, 184 to 186 are located above and belowbut laterally from the central regions of the first, second, third andfourth proof masses 17 a to 17 b. Of course, various other electrodearrangements could be used in other embodiments.

Referring to FIG. 12, the first driving electrode assembly 187 isconfigured to drive a vertical motion of each of the first and secondproof masses 17 a, 17 b along the first axis at an out-of-plane drivefrequency. For example, the drive signal can be a time-periodicsinusoidal signal. Depending on the application, the out-of-plane drivefrequency may or not correspond to a resonant frequency of the resonantstructures 54 formed by each of the first and second proof masses 17 a,17 b and their associated spring assemblies. In the illustratedembodiment, the first driving electrode 187 assembly is configured todrive the first proof mass 17 a 180 degrees out-of-phase relative to thesecond proof mass 17 b. For this purpose, the first driving electrodeassembly 187 can include a pair of top driving electrodes 13 e, 13 f,one of which being located above a central region 171 of the first proofmass 17 a and the other being located above a central region 171 of thesecond proof mass 17 b, and a pair of bottom driving electrodes 15 e, 15f, one of which being located below the central region 171 of the firstproof mass 17 a and the other being located below the central region 171of the second proof mass 17 b.

Referring still to FIG. 12, the fourth sensing electrode assembly 184 isconfigured to sense a Coriolis-induced, rocking motion of the first andsecond proof masses 17 a, 17 b along the y axis, which is indicative ofan angular rate Ω_(x) about the x axis. In the illustrated embodiment,the fourth sensing electrode assembly 184 forms first and secondcapacitors with the first and second proof masses 17 a, 17 b andmeasures a difference between a capacitance of the first capacitor and acapacitance of the second capacitor. The capacitance difference isindicative of the angular rate Ω_(x) to be measured. To measure thiscapacitance difference, the fourth sensing electrode assembly 184 caninclude:

-   -   a first pair of top sensing electrodes 13 g, 13 h disposed along        a line parallel to the y axis, on opposite sides of the top        driving electrode 13 e;    -   a second pair of top sensing electrodes 13 i, 13 j disposed        along a line parallel to the y axis, on opposite sides of the        top driving electrode 13 f;    -   a first pair of bottom sensing electrodes 15 g, 15 h disposed        along a line parallel to the y axis, on opposite sides of the        bottom driving electrode 15 e; and    -   a second pair of bottom sensing electrodes 15 i, 15 j disposed        along a line parallel to the y axis, on opposite sides of the        bottom driving electrode 15 f;

Of course, the number and arrangement of electrodes can vary dependingon the application in which the MEMS motion sensor is to be used.

As the first and second proof masses 17 a, 17 b are driven vertically180 degrees out-of-phase by the first driving electrode assembly 187,their respective Coriolis-induced, rocking motions along the y axis whensubjected to angular rate about the x axis will also be 180 degreesout-of phase. It will be appreciated that by using two proof massesdriven 180 degrees out of phase, the induced Coriolis accelerations ofthe two proof masses will also be 180 degrees out of phase, whereas anylinear acceleration component will have the same effect on each mass.Thus when the signals from corresponding electrodes on the two massesare subtracted, any linear acceleration signals will cancel out.

In this regard, FIG. 12 depicts a snapshot in time of the first andsecond proof masses 17 a, 17 b at their maximum amplitude displacementpoints. Therefore, by synchronously measuring the difference incapacitances of electrodes on similar sides of the two proof masses 17a, 17 b (e.g., 13 g and 13 i, or 13 h and 13 j), the time-dependentcapacitance change due to the angular rate around the x axis is obtainedsince the angular rate signals (C₀+/−ΔC_(Coriolis)) on these twoelectrodes are of opposite sign while the static or low-frequencyresponses due to y and z acceleration (C₀+ΔC_(x)+ΔC_(z)), being of thesame sign, are subtracted out. Of course, other electrode configurationsinvolving or not differential capacitance measurements can be used inother embodiments.

It is to be noted that by proper selection or adjustment of themechanical and/or geometrical properties of the first and second proofmasses 17 a, 17 b and their associated spring assemblies, the resonantfrequencies of the oscillation modes involved in the measurement of theangular rate Ω_(x) about the x axis can be tailored to provide eithermatched or nearly matched resonance conditions between the driving andsensing modes, where the driving and sensing resonant frequencies of thedriving and sensing modes are equal or close to each other, or unmatchedresonance conditions between the driving and sensing modes, wheredriving and sensing resonant frequencies are substantially differentfrom each other.

Referring back to FIGS. 8 to 10, the fifth sensing electrode assembly185 is configured to sense a Coriolis-induced, rocking motion of thefirst and second proof masses 17 a, 17 b along the x axis, which isindicative of an angular rate Ω_(y) about the y axis. In the illustratedembodiment, the fifth sensing electrode assembly 185 forms third andfourth capacitors with the first and second proof masses 17 a, 17 b andmeasures a difference between a capacitance of the third capacitor and acapacitance of the fourth capacitor. The capacitance difference isindicative of the angular rate Ω_(y) to be measured. To measure thiscapacitance difference, the fifth sensing electrode assembly 185 caninclude:

-   -   a first pair of top sensing electrodes 13 k, 13 l disposed along        a line parallel to the x axis, on opposite sides of the top        driving electrode 13 e;    -   a second pair of top sensing electrodes 13 m, 13 n disposed        along a line parallel to the x axis, on opposite sides of the        top driving electrode 13 f;    -   a first pair of bottom sensing electrodes 15 k, 15 l disposed        along a line parallel to the x axis, on opposite sides of the        bottom driving electrode 15 e; and    -   a second pair of bottom sensing electrodes 15 m, 15 n disposed        along a line parallel to the x axis, on opposite sides of the        bottom driving electrode 15 f;

Of course, the number and arrangement of electrodes can vary dependingon the application in which the MEMS motion sensor is to be used.

For this configuration of the fifth sensing electrode assembly 185, theangular rate Ω_(y) about the y axis can be determined using differentialcapacitance measurements involving the first and second proof masses 17a, 17 b being driven 180 degrees out-of-phase from each other, as inFIG. 12 for Ω_(x). Of course, other electrode configurations involvingor not differential capacitance measurements can be used in otherembodiments. Also, like in the measurement of Ω_(x), the measurement ofthe angular rate Ω_(y) can be performed either with matched or nearlymatched driving and sensing modes or with unmatched driving and sensingmodes.

Referring now to FIGS. 13, 14A and 14B, it will be understood that forthe illustrated embodiment of the MEMS motion sensor 10, the angularrate Ω_(z) around the z axis is to be measured differently than aroundthe x and y axes since the drive axis has to be perpendicular to theaxis about which the angular rate is to be sensed. Accordingly, in FIG.13, the second driving electrode assembly 188 is configured to drive arocking motion of each of the third and fourth proof masses 17 c, 17 dalong the y axis at an in-plane drive frequency. In this regard, FIG. 13depicts a snapshot in time of the driven third and fourth proof masses17 c, 17 d at their maximum driven amplitude displacement points alongthe y axis. Depending on the application, the in-plane drive frequencymay or not correspond to a resonant frequency of the resonant structures54 formed by each of the first and second proof masses 17 c, 17 d andtheir associated spring assemblies. In the illustrated embodiment, thesecond driving electrode assembly 188 is configured to drive the thirdproof mass 17 c 180 degrees out-of-phase relative to the fourth proofmass 17 d. For this purpose, the second driving electrode assembly 188can include:

-   -   a first pair of top driving electrodes 13 o, 13 p disposed along        a line parallel to y axis, above and laterally offset with        respect to a central region 171 of the third proof mass 17 c;    -   a second pair of top driving electrodes 13 q, 13 r disposed        along a line parallel to the y axis, above and laterally offset        with respect to a central region 171 of the fourth proof mass 17        d;    -   a first pair of bottom driving electrodes 15 o, 15 p disposed        along a line parallel to y axis, below and laterally offset with        respect to the central region 171 of the third proof mass 17 c;        and    -   a second pair of bottom driving electrodes 15 q, 15 r disposed        along a line parallel to the y axis, below and laterally offset        with respect to the central region 171 of the fourth proof mass        17 d.

Referring still to FIG. 13, in order to drive the third and fourth proofmasses 17 c, 17 d 180 degrees out-of-phase with each other using theillustrated configuration illustrated for the second driving electrodeassembly 188, the vertically aligned top and bottom driving electrodeslocated above and below one side of the third proof mass 17 c (e.g.,driving electrodes 13 o, 15 o) are driven 180 degrees out-of-phaserelative to the vertically aligned top and bottom driving electrodeslocated above and below the other side of the third proof mass 17 c(e.g., driving electrodes 13 p, 15 p) as well as 180 degreesout-of-phase relative to their counterparts on the fourth proof mass 17d (e.g., driving electrodes 13 q, 15 q). Likewise, the verticallyaligned top and bottom driving electrodes located above and below oneside of the fourth proof mass 17 d (e.g., driving electrodes 13 r, 15 r)are driven 180 degrees out-of-phase relative to the vertically alignedtop and bottom driving electrodes located above and below the other sideof the fourth proof mass 17 d (e.g., driving electrodes 13 q, 15 q) aswell as 180 degrees out-of-phase relative to their counterparts on thethird proof mass 17 c (e.g., driving electrodes 13 p, 15 p).

Turning now to FIGS. 14A and 14B, the sixth sensing electrode assembly186 is configured to sense a Coriolis-induced, rocking motion of thethird and fourth proof masses 17 c, 17 d along the x axis, which isindicative of an angular rate Ω_(z) about the z axis. In the illustratedembodiment, the sixth sensing electrode assembly 186 forms fifth andsixth capacitors with the third and fourth proof masses 17 c, 17 d andmeasures a difference between a capacitance of the fifth capacitor and acapacitance of the sixth capacitor. The difference is indicative of theangular rate Ω_(z) to be measured. To measure this capacitancedifference, the six sensing electrode assembly 186 can include:

-   -   a first pair of top sensing electrodes 13 s, 13 t disposed along        a line parallel to the x axis, above and laterally offset with        respect to a central region 171 of the third proof mass 17 c;    -   a second pair of top sensing electrodes 13 u, 13 v disposed        along a line parallel to the x axis, above and offset with        respect to a central region 171 of the fourth proof mass 17 d;    -   a first pair of bottom sensing electrodes 15 s, 15 t disposed        along a line parallel to the x axis, below and laterally offset        with respect to a central region 171 of the third proof mass 17        c; and    -   a second pair of bottom sensing electrodes 15 u, 15 v disposed        along a line parallel to the x axis, below and offset with        respect to a central region 171 of the fourth proof mass 17 d.

As the first and second proof masses 17 a, 17 b are driven vertically180 degrees out-of-phase by the first driving electrode assembly 187,their respective Coriolis-induced, rocking motions along the x axis whensubjected to angular rate about the z axis will also be 180 degreesout-of phase. In this regard, FIGS. 14A and 14B depict a snapshot intime of the third and fourth proof masses 17 c, 17 d, respectively attheir maximum amplitude displacement points along the x axis. Therefore,by synchronously measuring the difference in capacitances of electrodesin similar positions on the two masses 17 c, 17 d (e.g., 13 s and 13 u,or 13 t and 13 v, or 15 u and 15 s, or 15 t and 15 v) the time-dependentcapacitance change due to the angular rate around the z axis is obtainedsince the angular rate signals (C₀+/−ΔC_(Coriolis)) on the twoelectrodes are of opposite sign while the static or low-frequencyresponses due to y and z acceleration (C₀+ΔC_(x)+ΔC_(z)), being of thesame sign, are subtracted out. Of course, other electrode configurationsinvolving or not differential capacitance measurements can be used inother embodiments. Also, the measurement of the angular rate Ω_(y) canbe performed either with matched or nearly matched driving and sensingmodes or with unmatched driving and sensing modes.

In some embodiments of the MEMS motion sensor the out-of-plane andin-plane driving frequencies each range from 1 to 100 kilohertz, andwherein each of the first, second and third sensing electrode assembliesare configured to sense the motion of the one or more proof massesassociated therewith at an acceleration sensing frequency that is lessthan between about 30 percent and 50 percent of both the out-of-planeand in-plane driving frequencies.

Electrical Contacts and Insulated Conducting Pathways

Referring back to FIGS. 8 to 10, the MEMS motion sensor 10 furtherincludes first and second sets of electrical contacts 42 a, 42 bprovided on the top cap wafer 12. The first set of electrical contacts42 a are electrically connected to the top electrodes 13, and the secondset of electrical contacts 42 b are electrically connected to the bottomelectrodes 15 by way of insulated conducting pathways 33. The insulatingconducting pathways 33 extend upwardly from the bottom electrodes 15successively through the bottom cap wafer 14, the frame structure 50 ofthe MEMS wafer 16 and the top cap wafer 12 until they reach theelectrical contacts 42 b of the second set. Of course, other electricalcontacts may be provided on the top cap wafer, such as, for example,connecting feedthroughs extending from the bottom cap wafer to the topcap wafer. Likewise, other insulated conducting pathways may be providedto connect electrodes of the top cap wafer 12, and also possibly of oneor more of the proof masses 17 a to 17 e.

Referring now to FIG. 15, there are illustrated different insulatedconducting pathways 33 a to 33 e provided in the MEMS motions sensor 10using 3DTCV architecture. A first insulated conducting pathway 33 aelectrically connects the outer or peripheral regions of the bottom capwafer 14, the handle layer 22, the device layer 20 and the top cap wafer12 to provide a case ground. A second insulating conducting pathway 33 bfeeds signals from bottom electrodes 15, upwardly through electricallyisolated feedthroughs in the MEMS wafer 16 and top cap wafer 12, to bondpads 42 b on top of the MEMS device 10. This pathway 33 b usesconducting vias 30 between the handle layer 22 and the device layer 20,through the insulating layer 24. The conducting vias 30 can also be usedto electrically connect the flexible springs 270 to the proof masses 17a to 17 e so that they are not floating electrically.

A third insulated conducting pathway 33 c brings signals to and from thedevice layer 20 while isolating it from the handle layer 22 in theperiphery of the MEMS device 10 by eliminating conducting vias alongthat path. This pathway 33 c may be useful, for example, for makingelectrical connections to the flexible springs 270 without shorting thesprings 270 to the handle layer 22. A fourth insulated conductingpathway 33 d to connect top electrodes 13 in the top cap wafer 12 toelectrical contacts 42 a (e.g., bond pads) on top of the MEMS device 10,while electrically isolating the top electrodes 13 from the rest of theMEMS device 10. A fifth insulated conducting pathway 33 routes signalsfrom top cap electrodes 13 to bottom cap electrodes to drive electrodesin parallel as illustrated in FIG. 13, or to pass signals through theMEMS device 10, for example, from an IC on top through the MEMS to anunderlying IC or printed circuit board (PCB).

Driving and Sensing Means

In addition, depending of the intended implementation of the MEMS motionsensor some of the top and bottom electrodes can be connectable todriving means, while other ones of the top and bottom electrodes can beconnectable to sensing means.

Alternatively, the top and bottom electrodes can also be reconfigurablyconnectable to driving and sensing means, for switching between driveand sense modes. As used herein, the terms “driving means” and “sensingmeans” refer broadly to any electronic circuitry configured to deliverelectrical signals to and receive electrical signals from the electrodeassembly in order to drive and sense the motion of the proof mass of theMEMS motion sensor, respectively.

Other Exemplary Embodiments of a MEMS Motion Sensor

Referring now to FIGS. 16A to 16C, three other exemplary embodiments ofa multi-mass MEMS motion sensor are illustrated. This embodiment sharesmany similarities with the embodiment described above and illustrated inFIGS. 8 to 15 in that they generally include a MEMS wafer provided witha frame structure, a plurality of proof masses and associated springassemblies, top cap and bottom cap wafers sandwiching the MEMS wafer,top and bottom electrodes forming capacitors with and detecting themotions of the plurality of proof masses, and the top and bottomelectrodes being together configured to detect motions of the pluralityof proof masses. However, the number of proof masses and their functionsin FIGS. 16A to 16C differ from those in FIGS. 8 to 15.

First, in FIG. 16A, the MEMS motion sensor 10 includes two proof masses17 a, 17 b. In this embodiment, the first proof mass 17 a is associatedwith the first driving electrode assembly 187 and the first, second,third, fourth and fifth sensing electrode assemblies 181 to 185, whilethe second proof mass 17 b is associated with the second drivingelectrode assembly 188 and the sixth sensing electrode assembly 186.Accordingly, the first proof mass 17 a is used for linear accelerationmeasurements along the x, y and z axes and in-plane angular ratemeasurements around the x and y axes, while the second proof mass 17 bis used for out-of-plane angular rate measurements around the z axis.

Second, in FIG. 16B, the MEMS motion sensor 10 includes three proofmasses 17 a to 17 c. In this embodiment, the first proof mass 17 a isassociated with the first driving electrode assembly 187 and the fourthand fifth sensing electrode assemblies 184, 185, the second proof mass17 b is associated with the second driving electrode assembly 188 andthe sixth sensing electrode assembly 186, and the third proof mass 17 cis associated with the first, second and third sensing electrodeassemblies 181 to 183. Accordingly, the first proof mass 17 a is usedfor in-plane angular rate measurements around the x and y axes, thesecond proof mass 17 b is used for out-of-plane angular ratemeasurements around the z axis, and the third proof mass 17 c is usedfor linear acceleration measurements along the x, y and z axes.

As in the embodiment described above with reference to FIGS. 8 to 15,the embodiment in FIG. 16B allows the in-plane angular rate measurementsto be separated from the out-of-plane angular rate measurements, andisolates the three-axis linear acceleration measurements to a separateproof mass 17 c to provide a different damping environment for theaccelerometer. However, in contrast to the embodiment illustrated inFIGS. 8 to 15, the embodiment in FIG. 16B does not involve using pairsof proof masses driven in anti-phase for eliminating or reducingunwanted acceleration signals in angular rate measurements.

Third, in FIG. 16C, the MEMS motion sensor 10 includes four proof masses17 a to 17 d. In this embodiment, the first and second proof masses 17a, 17 b are associated with the first driving electrode assembly 187 andthe fourth and fifth sensing electrode assemblies 184, 185, the thirdand fourth proof masses 17 c, 17 d are associated with the seconddriving electrode assembly 188 and the sixth sensing electrode assembly186, and at least one of the four proof masses 17 a to 17 d is furtherassociated with the first, second and third sensing electrode assemblies(e.g., the second proof mass 17 b in FIG. 16C). Accordingly, the firstand second proof masses 17 a are used for in-plane angular ratemeasurements around the x and y axes, the third and fourth proof masses17 b are used for out-of-plane angular rate measurements around the zaxis, and one of the four proof masses 17 a to 17 d is used for linearacceleration measurements along the x, y and z axes.

It is to be understood that while the embodiments illustrated in FIGS. 8to 16C includes between two and five proof masses, it is also possibleto provide, in other embodiments, a multiple-mass MEMS motion sensorincluding more than five proof masses, so as to increase accuracy and/orredundancy of the x, y or z acceleration and/or Ω_(x), Ω_(y) or Ω_(z)angular rate measurements.

Method of Measuring Acceleration and Angular Rate

In accordance with another aspect, there is provided a method ofmeasuring acceleration and angular rate along mutually orthogonal first,second and third axes. The method for acceleration and angularmeasurement will be described in conjunction with FIGS. 8 to 15, whichillustrate a multi-mass MEMS motion sensor according to an exemplaryembodiment. It will be understood, however, that there is no intent tolimit the method to this embodiment, for the method may admit to otherequally effective embodiments. In particular, it will be understood thatwhile the measurement method can, by way of example, be performed withthe multi-mass MEMS motion sensor like that described above withreference to FIGS. 8 to 15, it may also be performed with any othersuitable MEMS motion sensor provided with a plurality of proof masses.

Referring to FIGS. 8 to 10, the method first includes a step ofproviding a multi-mass MEMS motion sensor 10. The MEMS motion sensor 10forms a multi-wafer stack that includes a MEMS wafer 16 having opposedtop and bottom sides 161, 162, and top and bottom cap wafers 12, 14respectively bonded to the top and bottom sides 161, 162 of the MEMSwafer 16. The top cap, bottom cap and MEMS wafer 12, 14, 16 areelectrically conductive. The MEMS motion sensor 10 may also include topand bottom electrodes respectively provided in the top and bottom capwafers 12, 14. The MEMS wafer 16 includes a frame structure 50, aplurality of proof masses 17 a to 17 e, and a plurality of springassemblies 27 a to 27 e each suspending a corresponding one of the proofmasses 17 a to 17 e from the frame structure 50 and enabling thecorresponding one of the proof masses 17 a to 17 e to move alongmutually orthogonal first, second and third axes z, x and y. The top capwafer 12, bottom cap wafer 14 and frame structure 50 together define oneor more cavities 31 such that each cavity 31 encloses one or more of theproof masses 17 a to 17 e, and each proof mass 17 a to 17 e is enclosedin one cavity 31. The top and bottom electrodes 13, 15 can formcapacitors with and are together configured to detect motions of theproof masses 17 a to 17 e.

In FIGS. 8 to 10, the MEMS motion sensor 10 is provided with five proofmasses 17 a to 17 e, where the first and second proof masses 17 a, 17 bare used for in-plane angular rate measurements around the x and y axes,the third and fourth proof masses 17 c, 17 d are used for out-of-planeangular rate measurements around the z axis, and the fifth proof mass 17e is used for linear acceleration measurements along the x, y and zaxes. Of course, configurations with two, three or four proof massescould be employed in other embodiments, as well as configurationincluding more than five proof masses, so as to increase accuracy and/orredundancy of the acceleration and/or angular rate measurements.

The method next includes a step of vibrating one or more of the proofmasses along the z axis at an out-of-plane drive frequency. For the MEMSmotion sensor 10 of FIGS. 8 to 10, this step involves vibrating thefirst and second proof masses 17 a, 17 b at the out-of-plane drivefrequency. Additionally, in some embodiment, the first and second proofmasses 17 a, 17 b may be vibrated 180 degrees out-of-phase with eachother. As mentioned above, by driving two proof masses driven 180degrees out of phase with each other, the induced Coriolis accelerationsof the two proof masses will also be 180 degrees out of phase, whereasany linear acceleration undergone by the two masses will be in-phase sothat the signals from corresponding electrodes on the two masses aresubtracted, any linear acceleration signals will cancel out.

The method also includes sensing Coriolis-induced, rocking motions alongthe y and x of the one or more second proof masses 17 a, 17 b drivenalong the z axis, in response to an angular rate about the x and y axes,respectively. For the MEMS motion sensor 10 of FIGS. 8 to 10, this stepinvolves sensing Coriolis-induced, rocking motions along the y and xaxes of the first and second proof masses 17 a, 17 b. This step canfurther involve forming first and third capacitors with the first proofmass 17 a and second and fourth capacitors with the second proof mass 17a, 17 b. A difference between a capacitance of the first capacitor and acapacitance of the second capacitor can be measured, this differencebeing indicative of the angular rate about the x axis. Likewise, adifference between a capacitance of the third capacitor and acapacitance of the fourth capacitor can also be measured, thisdifference being indicative of the angular rate of the first and secondproof masses 17 a, 17 b about the y axis.

The method further includes vibrating one or more of the proof masses ina rocking motion along the y axis at an in-plane drive frequency. Forthe MEMS motion sensor 10 of FIGS. 8 to 10, this step involves vibratingthe third and fourth proof masses 17 c, 17 d in a rocking motion alongthe y axis. Additionally, and as for the first and second proof masses17 a, 17 b, in some embodiment, the third and fourth proof masses 17 c,17 d may be vibrated 180 degrees out-of-phase with each other.

The method also includes sensing a Coriolis-induced, rocking motionalong the x axis of the one or more proof masses driven along the yaxis, in response to an angular rate about the z axis. For the MEMSmotion sensor 10, this step involves sensing a Coriolis-induced, rockingmotion along the x axis of the third and fourth proof masses 17 c, 17 d.This step can further involve forming fifth and sixth capacitorsrespectively with the third and fourth proof masses 17 c, 17 d. Adifference between a capacitance of the fifth capacitor and acapacitance of the sixth capacitor can be measured, this differencebeing indicative of the angular rate about the z axis.

The method also includes sensing a translational motion along the zaxis, a rotation about the x axis, and a rotation about the y axis ofone of the proof masses, indicative of linear accelerations along the z,x and y axes, respectively. For the MEMS motion sensor 10 of FIGS. 8 to10, the linear acceleration measurements involve only the fifth proofmass 17 e. In some embodiment, the step of sensing the translationalmotion along the z axis, the rotation about the x axis, and the rotationabout the y axis of the fifth proof mass 17 e is carried at respectiveacceleration sensing frequencies that are each less than between 30percent and 50 percent of both the out-of-plane and in-plane drivefrequencies.

Method for Manufacturing a Multi-Mass MEMS Motion Sensor

In accordance with another aspect, there is provided a method ofmanufacturing a MEMS motion sensor including a plurality of proofmasses. The method for manufacturing the MEMS device will be describedwith reference to the diagrams of FIGS. 17A to 17P, which schematicallyillustrate steps of an exemplary embodiment. It will be understood,however, that there is no intent to limit the method to this embodiment,for the method may admit to other equally effective embodiments. It willalso be understood that the manufacturing method can, by way of example,be performed to fabricate multi-mass MEMS motion sensors like thosedescribed above with reference to FIGS. 8 to 15 and 16A to 16C, or anyother suitable MEMS motion sensor provided with a plurality of proofmasses.

Referring to FIGS. 17A to 17P, there are schematically illustratedfabrication steps of an exemplary embodiment of the method formanufacturing a multi-mass MEMS motion sensor.

Referring to FIG. 17A, the manufacturing method includes a step ofproviding a top cap wafer 12. The top cap wafer 12 has opposed inner andouter sides 121, 122 and is made of an electrically conductivesemiconductor material such as, for example, a silicon-basedsemiconductor. In some embodiments, the step of providing the top capwafer 12 can include a step of forming recesses 51, or capacitor gaps,by removing top cap wafer material from a central region of the innerside 121 of the top cap wafer 12. The recesses 51 can be formed by anyof several manufacturing methods including, but not limited to,photolithography and etch or patterned oxidation and etch. The recesses51 may eventually form part of cavities whose role are to house proofmasses once the top cap wafer 12 is bonded to a MEMS wafer 16, asdescribed below. The manufacturing method also includes a step offorming top electrodes and insulated conducting cap wafer channels 123from the inner side 121 into the top cap wafer 12, which can includepatterning trenches 52 into the top cap wafer 12.

Turning to FIG. 17B, the trenches 52 may be filled with an insulatingmaterial 53. Alternatively, the trenches 52 may be lined with aninsulating material 53 and then filled with a conducting material 55.For this purpose, trench and fill processes are available at differentMEMS fabrication facilities, and the insulating and conducting materialsmay vary between them.

Referring to FIG. 17C, the steps of FIGS. 17A and 17B may be repeated ona bottom cap wafer 14 has opposed inner and outer sides 141, 142 to forma pattern of bottom cap electrodes, as well as recesses 51 or capacitorgaps, and insulated conducting cap wafer channels 143. As for the topcap wafer 12, the bottom cap wafer 14 is made of an electricallyconductive semiconductor material such as, for example, a silicon-basedsemiconductor.

Referring now to FIG. 17D, the manufacturing method next includesproviding a MEMS wafer 16 having opposed top and bottoms sides 161, 162.In FIG. 17D, the MEMS wafer is a SOI wafer including a device layer 20,a handle layer 22, and an insulating layer 24 (e.g., buried oxide)sandwiched between the device layer 20 and the handle layer 22.Conducting vias 30 are formed between the device layer 20 and the handlelayer 22 through the insulating layer 24. The conducting vias 30 arepatterned at desired spots on the device layer 20 and etched through thedevice layer 20 and insulating layer 24 to or slightly into the handlelayer 22. In FIG. 17E, the conducting vias 30 may then be filled with aconducting material 55 which can be doped polycrystalline silicon(polysilicon), a metal, or another suitable conducting material. In thisway an electrical path is formed vertically between the device layer 20and the handle layers 22 of the MEMS wafer 16.

Referring to FIG. 17F, MEMS patterns including leads 62 and feedthroughs63 may be patterned, delimited by trenches 64 in the device layer 20.

Referring to FIG. 17G, the manufacturing method includes a step ofaligning and bonding the top side 161 of the MEMS wafer 16 to the innerside 121 of the top cap wafer 12. This step can involve aligning thefeedthroughs 63 in the device layer 20 to corresponding pads on the topcap wafer 12, aligning the electrodes 13 in the top cap wafer 12 tocorresponding electrodes 65 on the MEMS wafer 16, aligning the insulatedconducting cap wafer channels 123 in the top cap wafer 12 withcorresponding portions of the insulated conducting MEMS channels 163.Advantageously, the wafer bonding process used can provide a conductivebond such as, for example, fusion bonding, gold thermocompressionbonding, or gold-silicon eutectic bonding.

Referring to FIG. 17H, the manufacturing method next includes patterningthe handle layer 22 of the MEMS wafer 16 with MEMS structures such as aplurality of proof masses 17 a, 17 b and feedthroughs 67, which arealigned with similar structures on the device layer 20, such as theelectrodes 65 and flexible springs (not shown). Trenches 69 can beetched around each feedthrough 67 to isolate the feedthrough 67 from therest of the handle layer 22. In some embodiments, if the feedthrough 67is attached to a SOI via on the device layer 20, then the feedthrough 67becomes an isolated electrical feedthrough. However, if the feedthrough67 is not attached to a SOI via, the feedthrough 67 then acts merely asa mechanical support.

Referring to FIG. 17I, the bottom cap wafer 14 may next be bonded to thebackside of the handle layer 22. Again, a wafer bonding method such asfusion bonding, gold thermocompression bonding, or gold-silicon eutecticbonding can be used to provide electrical contacts between thefeedthroughs 67 in the MEMS wafer 16 and pads 70 in the bottom cap wafer14, which are connected electrically to the bottom electrodes 15. Inthis manner, a conductive path can be provided from the bottomelectrodes 15 through the bottom cap pads 70, handle feedthroughs 67,SOI vias 30, and SOI device layer pads 63 to the top cap pads.

Referring to FIG. 17J, at this stage of the manufacturing method, theMEMS motion sensor 10 is hermetically sealed between the top and bottomcap wafers 12, 14 and the proof masses 17 a, 17 b are aligned with thetop cap and bottom cap electrodes 13, 15. Because the insulatingconducting pathways 33 do not yet fully penetrate the caps, theelectrodes on each cap are shorted together through the remainingsilicon (see, e.g., FIG. 17I). Both cap wafers 12, 14 may then bethinned, for example by grinding and polishing, to expose the insulatingconducting channels 64. The electrodes are thus electrically isolatedexcept for connections to top cap pads through the feedthroughs 67 andconducting vias 30. Both outer surfaces of the top and bottom cap wafers12, 14 can be passivated with an insulating oxide layer 71 forprotection purposes.

Referring to FIG. 17K, contacts 42 a can be opened over the pads in thetop cap wafer 12, bond pad metal 73 can be deposited and patterned, andpassivating oxide 74 can be applied and patterned to expose the bondpads. These or similar steps may be performed on the bottom cap wafer14. In this way, electrical connections from the top, sides, and/orbottom of the MEMS motion sensor can become accessible from the top orbottom for wire bonding, flip chip bonding, or wafer bonding. Aftercompletion of the step depicted in FIG. 17K, the wafer level fabricationof a hermetically sealed MEMS motion sensor wafer 72 is obtained.

At this point, if desired, the MEMS motion sensor wafer 72 can be dicedinto individual MEMS chips. Alternatively, the 3DTCV architecturedescribed herein may allow a wafer containing ICs for sensing and dataprocessing to be bonded directly to the MEMS motion sensor wafer 72. Thewafer-level integration of the 3D system (3DS) can involve bonding of anapplication-specific IC (ASIC) wafer designed with the appropriatesystem electronics for the application and with a physical bond padlayout commensurate with the MEMS motion sensor wafer 72.

Referring to FIG. 17L, bump-bonds 75 can be applied to one side of theMEMS motion sensor wafer 72. Numerous approaches and materials used inthe semiconductor industry can provide wafer bumps. In one embodiment, athick photoresist underfill 76 is applied to one side (e.g., the top capwafer 12 in FIG. 17L) of the MEMS motion sensor wafer 72 and patternedto produce an electroplating mask with openings over the bond pad metal73. Thick solder can be electroplated into the openings. The solder canbe left as a column or reflowed into balls, depending upon the bondingmethod. The photoresist can be stripped leaving the balls isolated, orcan be left in place as an underfill to protect the wafer surface. Ifthe photoresist is removed, a separate polymeric underfill layer 76 canbe coated and patterned around the solder balls. The purpose of theunderfill is to protect the wafer or chip surface and to mitigate bumpshearing due to the thermal stress of heating during bonding.

Referring to FIG. 17M, an ASIC wafer 77 can then be flipped and alignedto the MEMS motion sensor wafer 72 (e.g., to the top cap wafer 12 inFIG. 17M) such that ASIC bond pads 78 are aligned to the top cap solderbumps 75. The ASIC wafer has circuitry electrically connected to theMEMS motion sensor wafer 72, and in particular, to the first and secondsets of electrical contacts (42 a, 42 b) described above for routingsignals to and from the top and bottom electrodes 13, 15. The ASIC wafermay be bonded to the MEMS motion sensor wafer 72 using temperature andpressure to produce a 3DS wafer 82. At this point the ASIC wafer 77(e.g., a CMOS wafer) can be thinned, if desired, and passivated with aPECVD oxide 79.

Referring to FIG. 17N, an underfill 80 and bottom cap solder bumps 81may next be applied to the bottom cap 14 of the 3DS wafer 82. Theprocess is similar to that illustrated in FIG. 17L, except that in thecase of solder bumps, a lower eutectic point solder than that used forthe top bump-bonds is generally used to avoid releasing the top solderjoints. The top of the ASIC wafer 77 may be protected with an ASIC oxidepassivation 79 layer and the MEMS/ASIC interface may be protected by theunderfill 76. The bonded 3DS wafer 82 can finally be diced intoindividual 3DS components.

Referring to FIG. 17O, in some embodiments, the 3DS wafer 82 can bediced into self-packaged MEMS IMUs without requiring additional wirebonding or external packaging to.

Referring to FIG. 17P, the MEMS IMU 82 diced into self-packaged MEMSIMUs can then be directly solder bumped to leads 83 on a PCB 84.

Of course, numerous modifications could be made to the embodimentsdescribed above without departing from the scope of the presentinvention.

1. A micro-electro-mechanical system (MEMS) motion sensor (10) comprising: a MEMS wafer (16) having opposed top and bottom sides (161, 162) and comprising a frame structure (50), a plurality of proof masses (17 a to 17 e), and a plurality of spring assemblies (27 a to 27 e) each suspending a corresponding one of the proof masses (17 a to 17 e) from the frame structure (50) and enabling the corresponding one of the proof masses (17 a to 17 e) to move along mutually orthogonal first, second and third axes; top and bottom cap wafers (12, 14) respectively bonded to the top and bottom sides (161, 162) of the MEMS wafer (16), the top cap, bottom cap and MEMS wafers (12, 14, 16) being electrically conductive, the top cap wafer (12), bottom cap wafer (14) and frame structure (50) together defining one or more cavities (31) each enclosing one or more of the plurality of proof masses (17 a to 17 e), each proof mass (17 a to 17 e) being enclosed in one of the one or more cavities (31); top and bottom electrodes (13, 15) respectively provided in the top and bottom cap wafers (12, 14) and forming capacitors with the plurality of proof masses (17 a to 17 e), the top and bottom electrodes (13, 15) being together configured to detect motions of the plurality of proof masses (17 a to 17 e); and first and second sets of electrical contacts (42 a, 42 b) provided on the top cap wafer (12), the first set of electrical contacts (42 a) being electrically connected to the top electrodes (13), and the second set of electrical contacts (42 b) being electrically connected to the bottom electrodes (15) by way of insulated conducting pathways (33) extending successively through the bottom cap wafer (14), the frame structure (50) of the MEMS wafer (16) and the top cap wafer (12).
 2. The MEMS motion sensor (10) according to claim 1, wherein the MEMS motion sensor (10) is configured as a six-degree-of-freedom (6-DOF) motion sensor (10) enabling three-axis linear acceleration and angular rate measurements.
 3. The MEMS motion sensor (10) according to claim 2, wherein the top and bottom electrodes (13, 15) form a plurality of electrode assemblies (181 to 188) each electrode assembly (181 to 188) including at least one pair of the top and/or bottom electrodes (13, 15), the plurality of electrode assemblies (181 to 188) comprising: a first sensing electrode assembly (181) associated with one or more of the plurality of proof masses (17 a to 17 e) and configured to detect a translational motion of the one or more proof masses (17 a to 17 e) associated with the first sensing electrode assembly (181) along the first axis, the translational motion being indicative of a linear acceleration along the first axis; a second sensing electrode assembly (182) associated with one or more of the plurality of proof masses (17 a to 17 e) and configured to detect a rotation of the one or more proof masses (17 a to 17 e) associated with the second sensing electrode assembly (182) about the third axis, the rotation about the third axis being indicative of a linear acceleration along the second axis; a third sensing electrode assembly (183) associated with one or more of the plurality of proof masses (17 a to 17 e) and configured to detect a rotation of the one or more proof masses (17 a to 17 e) associated with the third sensing electrode assembly (183) about the second axis, the rotation about the second axis being indicative of a linear acceleration along the third axis; a first driving electrode assembly (187) associated with and configured to drive a motion of one or more associated ones of the plurality of proof masses (17 a to 17 e) along the first axis at an out-of-plane drive frequency; a fourth sensing electrode assembly (184) associated with the one or more proof masses (17 a to 17 e) associated with the first driving electrode assembly (187) and configured to sense a Coriolis-induced, rocking motion of the one or more proof masses (17 a to 17 e) associated with the first driving electrode assembly (187) along the third axis, the Coriolis-induced, rocking motion along the third axis being indicative of an angular rate about the second axis; a fifth sensing electrode assembly (185) associated with the one or more proof masses (17 a to 17 e) associated with the first driving electrode assembly (187) and configured to sense a Coriolis-induced, rocking motion of the one or more proof masses (17 a to 17 e) associated with the first driving electrode assembly (187) along the second axis, the Coriolis-induced, rocking motion along the second axis being indicative of an angular rate about the third axis; a second driving electrode assembly (188) associated with and configured to drive a rocking motion of one or more associated ones of the proof masses (17 a to 17 e) along one of the second and third axes at an in-plane drive frequency; and a sixth sensing electrode assembly (186) associated with the one or more proof masses (17 a to 17 e) associated with the second driving electrode assembly (188) and configured to sense a Coriolis-induced, rocking motion of the one or more proof masses (17 a to 17 e) associated with the second driving electrode assembly (188) along the other one of the second and third axes, the Coriolis-induced, rocking motion along the other one of the second and third axes being indicative of an angular rate about the first axis.
 4. The MEMS motion sensor (10) according to claim 3, wherein the out-of-plane and in-plane drive frequencies each range from 1 to 100 kilohertz, and wherein each of the first, second and third sensing electrode assemblies are configured to sense the motion of the one or more proof masses (17 a to 17 e) associated therewith at an acceleration sensing frequency that is less than between about 30 percent and 50 percent of both the out-of-plane and in-plane drive frequencies.
 5. The MEMS motion sensor (10) according to claim 3 or 4, wherein the cavity 31 of each mass associated with at least one of the first and second driving electrode assemblies (187, 188) is a hermetically sealed vacuum cavity.
 6. The MEMS motion sensor (10) according to any one of claims 3 to 5, wherein the plurality of proof masses (17 a to 17 e) consists of two proof masses (17 a, 17 b) configured as follows: a first proof mass (17 a) associated with the first driving electrode assembly (187) and the first, second, third, fourth and fifth sensing electrode assemblies (181 to 185); and a second proof mass (17 b) associated with the second driving electrode assembly (188) and the sixth sensing electrode assembly (186).
 7. The MEMS motion sensor (10) according to any one of claims 3 to 5, wherein the plurality of proof masses (17 a to 17 e) consists of three proof masses (17 a to 17 c) configured as follows: a first proof mass (17 a) associated with the first driving electrode assembly (187) and the fourth and fifth sensing electrode assemblies (184, 185); a second proof mass (17 b) associated with the second driving electrode assembly (188) and the sixth sensing electrode assembly (186); and a third proof mass (17 c) associated with the first, second and third sensing electrode assemblies (181 to 183).
 8. The MEMS motion sensor (10) according to any one of claims 3 to 5, wherein the plurality of proof masses (17 a to 17 e) consists of four proof masses (17 a to 17 d) configured as follows: first and second proof masses (17 a, 17 b) both associated with the first driving electrode assembly (187) and the fourth and fifth sensing electrode assemblies (184, 185); and third and fourth proof masses (17 c, 17 d) both associated with the second driving electrode assembly (188) and the sixth sensing electrode assembly (186), at least one of the four proof masses (17 a to 17 d) being further associated with the first, second and third sensing electrode assemblies (181 to 183).
 9. The MEMS motion sensor (10) according to any one of claims 3 to 5, wherein the plurality of proof masses (17 a to 17 e) consists of five proof masses (17 a to 17 e) configured as follows: first and second proof masses (17 a, 17 b) both associated with the first driving electrode assembly (187) and the fourth and fifth sensing electrode assemblies (184, 185); and third and fourth proof masses (17 c, 17 d) both associated with the second driving electrode assembly (188) and the sixth sensing electrode assembly (186); and a fifth proof mass (17 e) associated with the first, second and third sensing electrode assemblies (181 to 183).
 10. The MEMS motion sensor (10) according to claim 9, wherein the five proof masses (17 a to 17 e) are arranged in a common plane encompassing the second and third axes, the fifth proof mass (17 e) being located centrally and surrounded by the first, second, third and fourth proof masses (17 a to 17 d).
 11. The MEMS motion sensor (10) according to claim 9 or 10, wherein the first driving electrode assembly (187) is configured to drive the first proof mass (17 a) 180 degrees out-of-phase relative to the second proof mass (17 b), and wherein the second driving electrode assembly (188) is configured to drive the third proof (17 c) mass 180 degrees out-of-phase relative to the fourth proof mass (17 d).
 12. The MEMS motion sensor (10) according to any one of claims 9 to 11, wherein: the fourth sensing electrode assembly (184) is configured to form first and second capacitors with the first and second proof masses (17 a, 17 b), respectively, and to measure a difference between a capacitance of the first capacitor and a capacitance of the second capacitor, said difference being indicative of an angular rate of the first and second proof masses (17 a, 17 b) about the second axis; the fifth sensing electrode assembly (185) is configured to form third and fourth capacitors with the first and second proof masses (17 a, 17 b), respectively, and to measure a difference between a capacitance of the third capacitor and a capacitance of the fourth capacitor, said difference being indicative of an angular rate of the first and second proof masses (17 a, 17 b) about the third axis; and the sixth sensing electrode assembly (186) is configured to form fifth and sixth capacitors with the third and fourth proof masses (17 c, 17 d), respectively, and to measure a difference between a capacitance of the fifth capacitor and a capacitance of the sixth capacitor, said difference being indicative of an angular rate of the third and fourth proof masses (17 c, 17 d) about the first axis.
 13. The MEMS motion sensor (10) according to any one of claims 9 to 12, wherein: the first driving electrode assembly (187) comprises: a pair of top driving electrodes (13 e, 13 f), one (13 e) of which being located above a central region (171) of the first proof mass (17 a) and the other (13 f) being located above a central region (171) of the second proof mass (17 b); and a pair of bottom driving electrodes (15 e, 15 f), one (15 e) of which being located below the central region (171) of the first proof mass (17 a) and the other (15 f) being located below the central region (171) of the second proof mass (17 b); the fourth sensing electrode assembly (184) comprises: a first pair of top sensing electrodes (13 g, 13 h) disposed along a line parallel to the third axis, on opposite sides of the top driving electrode (13 e) located above the central region (171) of the first proof mass (17 a); a second pair of top sensing electrodes (13 i, 13 j) disposed along a line parallel to the third axis, on opposite sides of the top driving electrode (13 f) located above the central region (171) of the second proof mass (17 b); a first pair of bottom sensing electrodes (15 g, 15 h) disposed along a line parallel to the third axis, on opposite sides of the bottom driving electrode (15 e) located below the central region (171) of the first proof mass (17 a); and a second pair of bottom sensing electrodes (15 i, 15 j) disposed along a line parallel to the third axis, on opposite sides of the bottom driving electrode (15 f) located below the central region (171) of the second proof mass (17 b); and the fifth sensing electrode assembly (185) comprises: a first pair of top sensing electrodes (13 k, 13 l) disposed along a line parallel to the second axis, on opposite sides of the top driving electrode (13 e) located above the central region (171) of the first proof mass (17 a); a second pair of top sensing electrodes (13 m, 13 n), disposed along a line parallel to the second axis, on opposite sides of the top driving electrode located (13 f) above the central region (171) of the second proof mass (17 b); a first pair of bottom sensing electrodes (15 k, 15 l) disposed along a line parallel to the second axis, on opposite sides of the bottom driving electrode (15 e) located below the central region (171) of the first proof mass (17 a); and a second pair of bottom sensing electrodes (15 m, 15 n) disposed along a line parallel to the second axis, on opposite sides of the bottom driving electrode (15 f) located below the central region (171) of the second proof mass (17 b).
 14. The MEMS motion sensor (10) according to any one of claims 9 to 13, wherein: the second driving electrode assembly (188) comprises: a first pair of top driving electrodes (13 o, 13 p) disposed along a line parallel to the one of the second and third axes, above and laterally offset with respect to a central region (171) of the third proof mass (17 c); a second pair of top driving electrodes (13 q, 13 r) disposed along a line parallel to the one of the second and third axes, above and laterally offset with respect to a central region (171) of the fourth proof mass (17 d); a first pair of bottom driving electrodes (15 o, 15 p) disposed along a line parallel to the one of the second and third axes, below and laterally offset with respect to the central region (171) of the third proof mass (17 c); and a second pair of bottom driving electrodes (15 q, 15 r) disposed along a line parallel to the one of the second and third axes, below and laterally offset with respect to the central region (171) of the fourth proof mass (17 d); and the sixth sensing electrode assembly (186) comprises: a first pair of top sensing electrodes (13 s, 13 t) disposed along a line parallel to the other one of the second and third axes, above and laterally offset with respect to a central region (171) of the third proof mass (17 c); a second pair of top sensing electrodes (13 u, 13 v) disposed along a line parallel to the other one of the second and third axes, above and offset with respect to a central region (171) of the fourth proof mass (17 d); a first pair of bottom sensing electrodes (15 s, 15 t) disposed along a line parallel to the other one of the second and third axes, below and laterally offset with respect to a central region (171) of the third proof mass (17 c); and a second pair of bottom sensing electrodes (15 u, 15 v) disposed along a line parallel to the other one of the second and third axes, below and offset with respect to a central region (171) of the fourth proof mass (17 d).
 15. The MEMS motion sensor (10) according to any one of claims 1 to 14, wherein each proof mass (17 a to 17 e) and corresponding spring assembly (27 a to 27 e) form a resonant structure (54) configured to provided matched or near-matched resonance conditions for angular rate measurements.
 16. The MEMS motion sensor (10) according to any one of claims 1 to 15, wherein the top cap wafer (12), bottom cap wafer (14) and MEMS wafer (16) are each made of a silicon-based semiconductor.
 17. The MEMS motion sensor (10) according to claim 16, wherein the MEMS wafer (16) is a silicon-on-insulator (SOI) wafer including a device layer (20), a handle layer (22) under and spaced from the device layer (20), and an insulating layer (24) sandwiched between the device and handle layers (20, 22).
 18. The MEMS motion sensor (10) according to any one of claims 1 to 17, wherein: the proof masses (17 a to 17 e) each have a thickness and a polygonal cross-section respectively along and perpendicular to the first axis; and the spring assemblies (27 a to 27 e) each comprise flexible springs (270) mechanically connecting the corresponding proof mass to the frame structure (50), the flexible springs (270) each having a thickness along the first axis that is significantly less than the thickness of the corresponding proof mass (17 a to 17 e).
 19. The MEMS motion sensor (10) according to claim 18, wherein the thickness of each of the plurality of proof masses (17 a to 17 e) ranges from 10 to 1000 micrometers.
 20. A MEMS motion sensor system architecture comprising: a MEMS motion sensor (10) according to any one of claims 1 to 20; and an integrated circuit (IC) wafer (77) bonded to the top cap wafer (12) of the MEMS motion sensor (10), the IC wafer (77) having circuitry electrically connected to the MEMS motion sensor (10).
 21. The MEMS motion sensor system architecture according to claim 20, wherein the circuitry of the IC wafer (77) is electrically connected to the first and second sets of electrical contacts (42 a, 42 b) of the MEMS motion sensor (10) for routing signals to and from the top and bottom electrodes (13, 15).
 22. A method of measuring acceleration and angular rate along mutually orthogonal first, second and third axes, the method comprising: (a) providing a MEMS motion sensor (10) comprising a MEMS wafer (16) having opposed top and bottom sides (161, 162) and comprising a frame structure (50), a plurality of proof masses (17 a to 17 e), and a plurality of spring assemblies (27 a to 27 e) each suspending a corresponding one of the proof masses (17 a to 17 e) from the frame structure (50) and enabling the corresponding one of the proof masses (17 a to 17 e) to move along the first, second and third axes, and top and bottom cap wafers (12, 14) respectively bonded to the top and bottom sides (161, 162) of the MEMS wafer (16), the top cap, bottom cap and MEMS wafer (12, 14, 16) being electrically conductive, the top cap wafer (12), bottom cap wafer (14) and frame structure (50) together defining one or more cavities (31) each enclosing one or more of the plurality of proof masses (17 a to 17 e), each proof mass (17 a to 17 e) being enclosed in one of the one or more cavities (31); (b) vibrating one or more of the proof masses (17 a to 17 e) along the first axis at an out-of-plane drive frequency; (c) sensing Coriolis-induced, rocking motions along the third and second axes of the one or more proof masses (17 a to 17 e) driven along the first axis, in response to an angular rate about the second and third axes, respectively; (d) vibrating one or more of the proof masses (17 a to 17 e) in a rocking motion along one of the second and third axes at an in-plane drive frequency; (e) sensing a Coriolis-induced, rocking motion along the other one of the second and third axes of the one or more proof masses (17 a to 17 e) driven along the one of the second and third axes, in response to an angular rate about the first axis; and (f) sensing a translational motion along the first axis, a rotation about the second axis, and a rotation about the third axis of one of the proof masses (17 a to 17 e), indicative of linear accelerations along the first, third and second axes, respectively.
 23. The method according to claim 22, wherein the plurality of proof masses consists of five proof masses (17 a to 17 e), and wherein: step (b) comprises vibrating first and second ones of the five proof masses (17 a to 17 e) along the first axis at the out-of-plane drive frequency; step (c) comprises sensing Coriolis-induced, rocking motions along the third and second axes of the first and second proof masses (17 a, 17 b), in response to an angular rate of the first and second proof masses (17 a, 17 b) about the second and third axes, respectively; step (d) comprises vibrating third and fourth ones of the five proof masses (17 a to 17 e) in a rocking motion along the one of the second and third axes at the in-plane drive frequency; step (e) comprises sensing a Coriolis-induced, rocking motion along the other one of the second and third axes of the third and fourth proof masses (17 c, 17 d), in response to an angular rate of the third and fourth proof masses (17 c, 17 d) about the first axis; and step (f) comprises sensing a translational motion along the first axis, a rotation about the second axis, and a rotation about the third axis of a fifth one of the five proof masses (17 a to 17 e), indicative of linear accelerations along the first, third and second axes of the fifth proof mass (17 e), respectively.
 24. The method according to claim 23, wherein step (b) comprises vibrating the first and second proof masses (17 a, 17 b) 180 degrees out-of-phase with each other
 25. The method according to claim 24, wherein step (c) comprises: forming first and third capacitors with the first proof mass (17 a), and second and fourth capacitors with the second proof mass (17 b); measuring a difference between a capacitance of the first capacitor and a capacitance of the second capacitor, said difference being indicative of the angular rate of the first and second proof masses (17 a, 17 b) about the second axis; and measuring a difference between a capacitance of the third capacitor and a capacitance of the fourth capacitor, said difference being indicative of the angular rate of the first and second proof masses (17 a, 17 b) about the third axis.
 26. The method according to any one of claims 23 to 25, wherein step (d) comprises vibrating the third and fourth proof masses (17 c, 17 d) 180 degrees out-of-phase with each other.
 27. The method according to claim 26, wherein step (e) comprises: forming fifth and sixth capacitors respectively with the third and fourth proof masses (17 c, 17 d); measuring a difference between a capacitance of the fifth capacitor and a capacitance of the sixth capacitor, said difference being indicative of the angular rate of the third and fourth proof masses (17 c, 17 d) about the first axis.
 28. The method according to any one of claims 23 to 27, wherein step (f) comprises sensing the translational motion along the first axis, the rotation about the second axis, and the rotation about the third axis of the fifth proof mass (17 e) at respective acceleration sensing frequencies that are each less than between 30 percent and 50 percent of both the out-of-plane and in-plane drive frequencies. 